Conductor, power storage device, electronic device, and method for forming conductor

ABSTRACT

A novel electrode is provided. A novel power storage device is provided. A conductor having a sheet-like shape is provided. The conductor has a thickness of greater than or equal to 800 nm and less than or equal to 20 m. The area of the conductor is greater than or equal to 25 mm2 and less than or equal to 10 m2. The conductor includes carbon and oxygen. The conductor includes carbon at a concentration of higher than 80 atomic % and oxygen at a concentration of higher than or equal to 2 atomic % and lower than or equal to 20 atomic %.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. In particular, oneembodiment of the present invention relates to a semiconductor device, adisplay device, a light-emitting device, a power storage device, astorage device, a driving method thereof, or a manufacturing methodthereof. In particular, one embodiment of the present invention relatesto a power storage device and a manufacturing method thereof.

Note that a power storage device in this specification refers to everyelement and/or device having a function of storing electric power.

2. Description of the Related Art

In recent years, secondary batteries such as lithium-ion secondarybatteries, lithium-ion capacitors, and air cells have been activelydeveloped. In particular, demand for lithium-ion secondary batterieswith high output and high energy density has rapidly grown with thedevelopment of the semiconductor industry, for electronic devices, forexample, portable information terminals such as cell phones,smartphones, and laptop computers, portable music players, and digitalcameras; medical equipment; next-generation clean energy vehicles suchas hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-inhybrid electric vehicles (PHEVs); and the like. The lithium-ionsecondary batteries are essential as rechargeable energy supply sourcesfor today's information society.

Graphene has been attracting a great deal of attention because of itsexcellent conductivity and the like, and a large-scale production methodand the like have been searched. As described in Non-Patent Document 1,a compound obtained by reduction of graphene oxide (GO) is referred toas reduced GO (rGO) in some cases and the physical property thereof hasbeen attracting attention.

An increase in the capacity of a power storage device has been neededyear by year. For example, as described in Patent Document 1,development has been advanced in order to increase the capacity of apower storage device by devising its electrode.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2012-009414

Non-Patent Document

-   [Non-Patent Document 1] A. Bagri et al., “Structural evolution    during the reduction of chemically derived graphene oxide”, NATURE    CHEMISTRY, vol. 2, 2010, pp. 581-587.

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel electrode. Another object of one embodiment of the presentinvention is to provide a novel power storage device.

Another object of one embodiment of the present invention is to providean electrode with a high capacity. Another object of one embodiment ofthe present invention is to provide a power storage device with highenergy density.

Another object of one embodiment of the present invention is to providea flexible power storage device. Another object of one embodiment of thepresent invention is to provide a long-life power storage device.Another object of one embodiment of the present invention is to providea highly reliable power storage device. Another object of one embodimentof the present invention is to provide a power storage device in which areduction in characteristics is small.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a conductor. The conductorhas a sheet-like shape. The conductor has a thickness of greater than orequal to 800 nm and less than or equal to 20 m. The area of theconductor is greater than or equal to 25 mm² and less than or equal to10 m². The conductor includes carbon and oxygen. The conductor includesa portion including carbon at a concentration of higher than 80 atomic %and oxygen at a concentration of higher than or equal to 2 atomic % andlower than or equal to 20 atomic %.

In the above structure, the conductor preferably includes a portionwhose electrical conductivity is greater than or equal to 0.1 S/cm andless than or equal to 10⁷ S/cm.

In any of the above structures, the conductor preferably includes anaggregate of graphene. Here, “including an aggregate of graphene” meansto include a plurality sheets of graphene or a plurality of graphenecompounds.

In the above structure, the conductor includes a plurality of sheets ofgraphene including two or more and one hundred or less layers, and thelength in a direction of a long side of graphene is preferably greaterthan or equal to 50 nm and less than or equal to 100 m. Furthermore, ineach of the above structures, the interlayer distance between adjacentlayers in the graphene is preferably greater than or equal to 0.335 nmand less than or equal to 0.7 nm.

In any of the above structures, the concentration of oxygen included inthe conductor is preferably measured by X-ray photoelectronspectroscopy. Furthermore, in any of the above structures, the conductorpreferably includes a bond of carbon and oxygen.

In any of the above structures, the conductor preferably includes atleast one of an epoxy group, a carbonyl group, and a hydroxyl groupbonded to carbon.

In any of the above structures, the conductor preferably includes sulfurat a concentration of higher than or equal to 10 ppm and lower than orequal to 5%.

Another embodiment of the present invention is a power storage device.The power storage device includes a positive electrode and a negativeelectrode. One of the positive electrode and the negative electrodeincludes the conductor described in any one of the above structures anda layer containing an active material. The layer is in contact with atleast one of surfaces of the conductor.

Another embodiment of the present invention is a power storage device.The power storage device includes a positive electrode and a negativeelectrode. The positive electrode includes the conductor described inany one of the above structures.

Another embodiment of the present invention is a power storage device.The power storage device includes a positive electrode and a negativeelectrode. The positive electrode includes the conductor described inany one of the above structures and a layer. The layer is in contactwith at least one of surfaces of the conductor. The layer includes apositive electrode active material.

In the power storage device described in any one of the abovestructures, it is preferable that the conductor be stacked over aseparator and wound.

Another embodiment of the present invention is an electronic deviceincluding the power storage device described in any one of the abovestructures.

The conductor of one embodiment of the present invention can be formedin such a manner that a plurality of graphene compounds are stacked soas to partly overlap with each other. A sheet-like graphene compound inwhich a plurality of graphene compounds partly overlap with each otheris referred to as a graphene compound sheet in some cases.

For example, the conductor of one embodiment of the present inventioncan be formed in such a manner that a sheet having a thickness ofgreater than or equal to 50 nm and an area of greater than or equal to 1mm² is formed by stacking a plurality of sheets of graphene oxide so asto partly overlap with each other, and the sheet is subjected toreduction treatment. In the conductor, the concentration of carbon ispreferably higher than 80 atomic %, the concentration of oxygen ispreferably higher than or equal to 2 atomic % and lower than or equal to20 atomic %, and the interlayer distance is preferably greater than orequal to 0.335 nm and less than or equal to 0.7 nm.

Another embodiment of the present invention is a method for forming aconductor. In the method, a sheet having a thickness of greater than orequal to 50 nm and an area of greater than or equal to 1 mm² is formedby stacking a plurality of sheets of graphene oxide so as to partlyoverlap with each other and subjected to reduction treatment. In theconductor, the concentration of carbon is preferably higher than 80atomic %, the concentration of oxygen is preferably higher than or equalto 2 atomic % and lower than or equal to 20 atomic %, and the interlayerdistance is preferably greater than or equal to 0.335 nm and less thanor equal to 0.7 nm.

One embodiment of the present invention can provide a novel electrode.Another embodiment of the present invention can provide a novel powerstorage device.

Another embodiment of the present invention can provide an electrodewith a high capacity. Another embodiment of the present invention canprovide a power storage device with high energy density.

Another embodiment of the present invention can provide a flexible powerstorage device. Another embodiment of the present invention can providea power storage device with a long lifetime. Another embodiment of thepresent invention can provide a highly reliable power storage device.Another embodiment of the present invention can provide a power storagedevice in which a reduction in characteristics is small.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams illustrating part of a conductor.

FIG. 2 is a diagram illustrating a conductor of one embodiment of thepresent invention.

FIGS. 3A and 3B are diagrams each illustrating a graphene compound.

FIGS. 4A and 4B are diagrams each illustrating part of a conductor ofone embodiment of the present invention.

FIGS. 5A and 5B are diagrams each illustrating a spray dry apparatus.

FIGS. 6A and 6B are diagrams each illustrating part of a cross sectionof a power storage device.

FIGS. 7A and 7B are diagrams each illustrating part of a cross sectionof a power storage device.

FIGS. 8A and 8B are diagrams each illustrating part of a cross sectionof a power storage device.

FIGS. 9A and 9B are diagrams each illustrating part of a cross sectionof a power storage device.

FIGS. 10A and 10B are diagrams each illustrating a particle of oneembodiment of the present invention.

FIGS. 11A and 11B are diagrams illustrating part of a cross section ofan electrode.

FIG. 12 is a diagram illustrating a power storage device.

FIGS. 13A and 13B are diagrams illustrating a method for manufacturing apower storage device.

FIGS. 14A and 14B are diagrams illustrating a method for manufacturing apower storage device.

FIGS. 15A and 15B are diagrams illustrating a power storage device and amethod for manufacturing the power storage device.

FIGS. 16A and 16B are diagrams illustrating a power storage device.

FIG. 17 is a diagram illustrating a power storage device.

FIGS. 18A and 18B are cross-sectional views each illustrating a powerstorage device.

FIGS. 19A and 19B are diagrams each illustrating a power storage device.

FIG. 20 is a diagram illustrating a power storage device.

FIGS. 21A to 21C are diagrams each illustrating part of a cross sectionof a power storage device.

FIGS. 22A and 22B are diagrams each illustrating part of a cross sectionof a power storage device.

FIGS. 23A to 23C are diagrams each illustrating part of a cross sectionof a power storage device.

FIGS. 24A and 24B are diagrams illustrating a power storage device.

FIGS. 25A to 25C illustrate a curvature radius of a surface.

FIGS. 26A to 26D illustrate a curvature radius of a film.

FIGS. 27A and 27B are diagrams illustrating an example of a powerstorage system.

FIGS. 28A1, 28A2, 28B1, and 28B2 each illustrate an example of a powerstorage system.

FIGS. 29A and 29B each illustrate an example of a power storage system.

FIGS. 30A to 30G illustrate examples of electronic devices.

FIGS. 31A to 31C illustrate an example of an electronic device.

FIG. 32 illustrates an example of an electronic device.

FIGS. 33A and 33B illustrate examples of electronic devices.

FIG. 34 is a block diagram showing one embodiment of the presentinvention.

FIGS. 35A to 35C are schematic views illustrating one embodiment of thepresent invention.

FIG. 36 is a circuit diagram illustrating one embodiment of the presentinvention.

FIG. 37 is a circuit diagram illustrating one embodiment of the presentinvention.

FIGS. 38A to 38C are schematic views illustrating one embodiment of thepresent invention.

FIG. 39 is a block diagram showing one embodiment of the presentinvention.

FIG. 40 is a flow chart showing one embodiment of the present invention.

FIG. 41 is a photograph showing one embodiment of the present invention.

FIG. 42 is a photograph showing one embodiment of the present invention.

FIG. 43 is a photograph showing one embodiment of the present invention.

FIGS. 44A to 44C show XRD evaluation results.

FIGS. 45A and 45B show XPS analysis results.

FIGS. 46A and 46B show XPS analysis results.

FIGS. 47A and 47B show XPS analysis results.

FIGS. 48A and 48B show XPS analysis results.

FIGS. 49A and 49B show XRD evaluation results.

FIGS. 50A and 50B show FT-IR evaluation results.

FIGS. 51A and 51B are graphs each showing charge and dischargecharacteristics.

FIGS. 52A and 52B are graphs each showing charge and dischargecharacteristics.

FIGS. 53A to 53C are optical micrographs.

FIG. 54A is an optical micrograph and FIG. 54B shows a SEM observationresult.

FIGS. 55A and 55B are optical micrographs.

FIGS. 56A and 56B show SEM observation results.

FIG. 57 shows a cross-sectional TEM observation result.

FIG. 58 shows a cross-sectional TEM observation result.

FIG. 59 is a photograph showing an experimental result.

FIGS. 60A and 60B show XPS analysis results.

FIG. 61 is a top view illustrating one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and examples of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the description of theembodiments and examples and it is easily understood by those skilled inthe art that the mode and details can be changed variously. Accordingly,the present invention should not be interpreted as being limited to thedescription of the embodiments below.

Note that in drawings used in this specification, the sizes,thicknesses, and the like of components such as films, layers,substrates, regions are exaggerated for simplicity in some cases.Therefore, the sizes of the components are not limited to the sizes inthe drawings and relative sizes between the components.

Note that the ordinal numbers such as “first” and “second” in thisspecification and the like are used for convenience and do not denotethe order of steps, the stacking order of layers, or the like.Therefore, for example, description can be made even when “first” isreplaced with “second” or “third”, as appropriate. In addition, theordinal numbers in this specification and the like are not necessarilythe same as those which specify one embodiment of the present invention.

Note that in structures of the present invention described in thisspecification and the like, the same portions or portions having similarfunctions are denoted by common reference numerals in differentdrawings, and descriptions thereof are not repeated. Further, the samehatching pattern is applied to portions having similar functions, andthe portions are not especially denoted by reference numerals in somecases.

Note that in this specification and the like, a positive electrode and anegative electrode for a power storage device may be collectivelyreferred to as a power storage device electrode; in this case, the powerstorage device electrode refers to at least one of the positiveelectrode and the negative electrode for the power storage device.

Here, a charge rate and a discharge rate of a power storage battery willbe described. For example, in the case of charging a secondary batterywith a certain capacity X[Ah] at a constant current, a charge rate of 1C means the current value I [A] with which charging is terminated inexactly 1 h, and a charge rate of 0.2 C means I/5 [A](i.e., the currentvalue with which charging is terminated in exactly 5 h). Similarly, adischarge rate of 1 C means the current value I [A] with whichdischarging is ended in exactly 1 h, and a discharge rate of 0.2 C meansI/5 [A] (i.e., the current value with which discharging is ended inexactly 5 h).

Embodiment 1

In this embodiment, a graphene compound of one embodiment of the presentinvention and a conductor including the graphene compound of oneembodiment of the present invention are described.

Graphene has carbon atoms arranged in one atomic layer. A π bond existsbetween the carbon atoms. Graphene including two or more and one hundredor less layers is referred to as multilayer graphene in some cases.Graphene and multilayer graphene has a length of the major axis in asurface or a length in the longitudinal direction of greater than orequal to 50 nm and less than or equal to 100 m or greater than or equalto 800 nm and less than or equal to 50 m, for example.

In this specification and the like, a compound including graphene ormultilayer graphene as a basic skeleton is referred to as graphenecompound. The graphene compound includes graphene and multilayergraphene.

The graphene compound is described in detail below.

The graphene compound is, for example, a compound in which graphene ormultilayer graphene is modified with an atom other than a carbon atom oran atomic group including an atom other than a carbon atom.Alternatively, the graphene compound may be a compound in which grapheneor multilayer graphene is modified with an atomic group mainly includingcarbon, such as an alkyl group or an alkylene group. Note that an atomicgroup with which graphene or multilayer graphene is modified is referredto as a substituent, a functional group, a characteristic group, or thelike in some cases. In this specification and the like, the term “beingmodified” means that an atom other than a carbon atom or an atomic groupincluding an atom other than a carbon atom is introduced into graphene,multilayer graphene, a graphene compound, or graphene oxide (to bedescribed later) by a substitution reaction, an addition reaction, orthe other reaction.

Note that a front surface and a back surface of graphene may be modifiedwith different atoms or different atomic groups. In multilayer graphene,layers may be modified with different atoms or different atomic groups.

As an example of graphene modified with the atom or the atomic groupdescribed above, graphene or multilayer graphene modified with oxygen ora functional group containing oxygen can be given. Here, examples of afunctional group containing oxygen include an epoxy group, a carbonylgroup such as a carboxy group, a hydroxyl group, and a lactol group. Agraphene compound modified with oxygen or a functional group containingoxygen is referred to as graphene oxide in some cases. In thisspecification, graphene oxide includes multilayer graphene oxide.

Next, an example of a method for forming graphene oxide is described.Graphene oxide can be obtained by oxidation of graphene or multilayergraphene described above. Alternatively, graphene oxide can be obtainedby separation of layers of graphite oxide. Graphite oxide can beobtained by oxidation of graphite. Here, graphene oxide may be furthermodified with the atom or the atomic group described above.

Graphene oxide can be formed by any of a variety of synthesis methodssuch as a Hummers method, a modified Hummers method, and oxidation ofgraphite.

For example, the Hummers method and the modified Hummers method are eacha method for forming graphite oxide by oxidizing graphite such as flakegraphite. The obtained graphite oxide is graphite which is oxidized inplaces and thus to which a functional group such as a carbonyl group, acarboxy group, a hydroxyl group, or a lactol group is bonded. In thegraphite oxide, the crystallinity of the graphite is lost and thedistance between layers is increased. Therefore, the layers can beeasily separated by ultrasonic treatment or the like to obtain grapheneoxide.

Here, an example of a method for forming graphene oxide by the modifiedHummers method is described. A Hummers method is as follows: a sulfuricacid solution of potassium permanganate or the like is mixed intographite powder to cause oxidation reaction; thus, a mixed solutioncontaining graphite oxide is formed. Because of the oxidation of carbonin graphite, graphite oxide has a functional group such as an epoxygroup, a carbonyl group, a carboxy group, or a hydroxyl group.Accordingly, the interlayer distance in graphite oxide is longer thanthe interlayer distance in graphite. Then, ultrasonic vibration isapplied to the mixed solution containing the graphite oxide, so that thegraphite oxide whose interlayer distance is long can be cleaved toseparate graphene oxide and to form a dispersion liquid containing thegraphene oxide.

When graphene oxide is formed by the modified Hummers method, theobtained graphene oxide includes an element such as sulfur or nitrogenin some cases, for example.

The concentration of sulfur contained in a graphene compound of oneembodiment of the present invention is preferably 5% or lower, andfurther preferably 3% or lower, for example.

The graphene compound of one embodiment of the present inventionincludes sulfur at a concentration of higher than or equal to 10 ppm andlower than or equal to 5%, higher than or equal to 100 ppm and lowerthan or equal to 3%, or higher than or equal to 0.1% and lower than orequal to 3% in some cases, for example.

Here, the concentration of sulfur contained in the graphene compound canbe measured by elementary analysis such as X-ray photoelectronspectroscopy (XPS), for example.

The graphene compound of one embodiment of the present inventionincludes nitrogen at a concentration of higher than or equal to 0.1% andlower than or equal to 3% in some cases, for example.

A compound that can be obtained by reducing graphene oxide is referredto as reduced graphene oxide (RGO) in some cases. Here, in some cases,RGO is expressed as “rGO” as described in Non-Patent Document 1. In RGO,in some cases, all oxygen atoms contained in the graphene oxide are notextracted and some oxygen atoms remain in a state where oxygen or anatomic group containing oxygen is bonded. In some cases, RGO includes afunctional group, e.g., an epoxy group, a carbonyl group such as acarboxy group, or a hydroxyl group.

A graphene compound may have a sheet-like shape where a plurality ofgraphene compounds partly overlap each other. Such a graphene compoundis referred to as a graphene compound sheet in some cases. The graphenecompound sheet has, for example, an area with a thickness larger than orequal to 0.33 nm and smaller than or equal to 10 mm, or preferablylarger than 0.34 nm and smaller than or equal to 10 m. The graphenecompound sheet may be modified with an atom other than carbon, an atomicgroup containing an atom other than carbon, an atomic group mainlycomposed of carbon such as an alkyl group, or the like. A plurality oflayers in the graphene compound sheet may be modified with differentatoms or atomic groups.

A graphene compound may have a five-membered ring composed of carbonatoms or a poly-membered ring that is a seven or more-membered ringcomposed of carbon atoms, in addition to a six-membered ring composed ofcarbon atoms. In the neighborhood of a poly-membered ring that is aseven or more-membered ring, a region through which an ion can pass maybe generated. As an example of an ion, a lithium ion can be given.Furthermore, an ion of an alkali metal other than a lithium ion; ananion and a cation used for an electrolyte; an anion and a cationcontained in an electrolyte solution; and the like can be given.

A plurality of graphene compounds may be gathered to form a sheet-likeshape. A graphene compound has a planar shape, thereby enabling surfacecontact.

In some cases, a graphene compound has high conductivity even when it isthin. The contact area between graphene compounds or between a graphenecompound and an active material can be increased by surface contact.Thus, even with a small amount of a graphene compound per volume, aconductive path can be formed efficiently.

A graphene compound may also be used as an insulator. For example, agraphene compound sheet may be used as a sheet-like insulator. Grapheneoxide, for example, has a higher insulation property than a graphenecompound that is not oxidized in some cases. A graphene compoundmodified with an atomic group may have an improved insulation property,depending on the type of the modifying atomic group.

FIG. 3A is a schematic view illustrating single-layer graphene. FIG. 3Billustrates an example of graphene modified with functional groups suchas an epoxy group, a carboxy group, and a hydroxyl group. Note that FIG.3B is merely an example, and graphene modified with functional groups isnot limited to this example.

<Conductor>

FIG. 2 is an external view of a conductor 201 of one embodiment of thepresent invention. The conductor 201 preferably has a sheet-like shape.

The conductor 201 preferably includes a graphene compound.

The thickness of the conductor 201 is greater than or equal to 0.33 nmand less than or equal to 100 m, greater than or equal to 50 nm and lessthan or equal to 100 m, or greater than or equal to 800 nm and less thanor equal to 20 m, for example.

The area of the conductor 201 is larger than or equal to 1 mm² andsmaller than or equal to 100 m², larger than or equal to 25 mm² andsmaller than or equal to 10 m², or larger than or equal to 100 mm² andsmaller than or equal to 3 m², for example.

The conductor 201 is described in detail below. A region 202 surroundedby a dashed line in FIG. 2 is part of a cross section of the conductor201. FIG. TA is an enlarged view of the region 202. FIG. 1B is anenlarged view of a region 202 a illustrated in FIG. TA. It is preferablethat the region 202 a include a plurality of graphene compounds 211 andthat the plurality of graphene compounds 211 partly overlap with eachother. A sheet having one sheet-like shape formed of a plurality ofgraphene compounds that partly overlap with each other is referred to asa graphene compound sheet in some cases. A graphene compound sheet ispreferably used as the conductor 201.

The length of one side (also referred to as a flake size) of thegraphene compound is greater than or equal to 50 nm and less than orequal to 100 m, preferably greater than or equal to 800 nm and less thanor equal to 20 m.

In a graphene compound sheet in which a plurality of graphene compoundsoverlap with each other, a region through which an ion can pass betweenadjacent graphene compounds may be generated, for example. Accordingly,a graphene compound sheet may have high ionic conductivity. A graphenecompound sheet may adsorb an ion easily.

It is considered that a graphene compound sheet in which a plurality ofgraphene compounds overlap with each other may be changed in shape inthe case where external force is applied such that graphene compoundsoverlapping in a planar manner slide on each other and thus less likelyto be cracked.

FIG. 61 illustrates an example of a top view illustrating part of theconductor 201. FIG. 61 illustrates an example of a state in which theplurality of graphene compounds 211 overlap with each other. Thus, asurface of the conductor 201 may have a step corresponding to thethickness of the graphene compound. Furthermore, the surface of theconductor 201 may have a region which is substantially surrounded by astep and flatter than the step. The area of the region may correspond tothe area of the graphene compound.

FIGS. 4A and 4B each illustrate an example of the region 202 a differentfrom the region 202 a in FIG. 1B.

The conductor 201 can be flexible and likely to be changed in shape byincluding a graphene compound. The conductor 201 may have highermechanical strength by including a graphene compound.

The conductor 201 can have higher conductivity by including graphene ormultilayer graphene as a graphene compound.

The electrical conductivity of the conductor 201 of one embodiment ofthe present invention is preferably greater than or equal to 0.1 S/cmand less than or equal to 10⁷ S/cm, further preferably greater than orequal to 1 S/cm and less than or equal to 10⁶ S/cm, and still furtherpreferably greater than or equal to 10 S/cm and less than or equal to10⁶ S/cm. In the case where the conductor 201 has a sheet-like shape,the electrical conductivity of the conductor 201 can be measured by afour-terminal method in which terminals are touched to a sheet surface.

An example of the interlayer distance in a graphene compound 211included in the conductor 201 is described. The interlayer distance inthe graphene compound 211 is, for example, longer than or equal to 0.335nm and shorter than or equal to 0.7 nm, longer than 0.34 nm and shorterthan or equal to 0.6 nm, longer than 0.34 nm and shorter than or equalto 0.5 nm, or longer than 0.34 nm and shorter than 0.44 nm. Examples ofa method for calculating the interlayer distance in the graphenecompound 211 include XRD and TEM.

In observation with a TEM, a small region, e.g.: a several-nanometer toseveral-micrometer square region, is observed. In XRD evaluation,average data on a larger region can be evaluated in some cases.

Next, the proportion of oxygen included in the conductor 201 can bemeasured by X-ray photoelectron spectroscopy (XPS), EDX, or the like.The proportion of oxygen included in the conductor 201 which is measuredby XPS is higher than or equal to 2 atomic % and lower than or equal to20 atomic %, preferably higher than or equal to 2 atomic % and lowerthan or equal to 11 atomic %, and further preferably higher than orequal to 3 atomic % and lower than or equal to 10 atomic %, with respectto the whole conductor 201. In the case where the conductor 201 isanalyzed by XPS and the spectrum of binding energy of carboncorresponding to C1s is subjected to waveform separation, the proportionof peaks indicating sp² with respect to the whole spectrum of C1s can beestimated as an area ratio. The proportion of sp² in the conductor 201is preferably higher than or equal to 50% and lower than or equal to 90%with respect to the whole spectrum of C1s.

The proportion of carbon included in the conductor 201 is preferablyhigher than 80% with respect to the whole conductor 201. The proportionof carbon can be measured by XPS, EDX, or the like, for example.

The proportion of oxygen in the graphene compound 211 included in theconductor 201, which is measured by XPS, is higher than or equal to 2atomic % and lower than or equal to 20 atomic %, preferably higher thanor equal to 2 atomic % and lower than or equal to 11 atomic %, andfurther preferably higher than or equal to 3 atomic % and lower than orequal to 10 atomic %, with respect to the whole conductor 201. In a bondof carbon in the graphene compound 211, the proportion of a double bondof carbon is preferably higher than or equal to 50% and lower than orequal to 90%, for example.

In the case where the proportion of a double bond of carbon is analyzedby XPS, for example, the spectrum of binding energy of carboncorresponding to C1s is subjected to waveform separation, so that theproportion of peaks indicating sp² with respect to the whole spectrum ofC1s can be estimated as an area ratio in some cases. Alternatively, inthe case where a ¹³C NMR spectrum is evaluated, a peak indicating adouble bond of carbon in a chemical shift of 130 ppm to 140 ppm or inthe vicinity thereof can be observed, for example. Alternatively, in thecase where a ¹³C NMR spectrum is evaluated, a peak indicating a C—O—Cbond in a chemical shift of 50 ppm to 60 ppm or in the vicinity thereofor a peak indicating a bond of carbon and a hydroxyl group in a chemicalshift of 70 ppm to 80 ppm or in the vicinity thereof can be observed,for example.

The peaks indicating sp² may be obtained as the proportion of the areaof the peak indicating the double bond of carbon with respect to thearea of all peaks indicating a bond of carbon which are observed by NMR,for example. For example, the peaks indicating sp² may be obtained asthe proportion of the area of the peak indicating the double bond ofcarbon with respect to the area of all peaks in a range of −50 ppm to250 ppm.

The proportion of carbon in the graphene compound 211 included in theconductor 201 is preferably higher than 80% with respect to the wholeconductor 201, for example. The proportion of carbon can be measured byXPS, EDX, or the like, for example.

<Intercalation Compound>

An intercalation compound including molecules or ions between layers maybe used as the graphene compound included in the conductor 201. In thecase where the graphene compound is an intercalation compound, theelectrical conductivity may be changed depending on the kind of amolecule or an ion included between layers. For example, the electricalconductivity of the graphene compound may be improved. The interlayerdistance may be increased depending on the size and the content of amolecule or an ion included between the layers.

<Method for Forming Conductor>

An example of a method for forming the conductor 201 including thegraphene compounds 211 is described below.

First, a graphene compound sheet 222 is formed. The graphene compoundsheet 222 can be formed using a graphene compound as a raw material by amethod such as a spray drying method or a coating method. Here, as anexample, the graphene compound sheet 222 including graphene oxide isformed by a spray drying method using a graphene oxide dispersion liquidas a raw material. Here, at least part of the graphene oxide included inthe graphene oxide dispersion liquid may be multilayer graphene oxide.

The graphene oxide dispersion liquid is used as a raw material, and aplurality of sheets of graphene oxide are formed over a plate by a spraydrying method, so that the graphene compound sheet 222 including thegraphene oxide can be obtained. Note that the spray drying method issuitable for manufacture of the graphene compound sheet of oneembodiment of the present invention because the thickness of an obtainedgraphene compound sheet can be controlled in some cases by adjustment ofthe deposition time, the concentration of dispersion liquid, or thelike. The obtained graphene compound sheet may be separated from theplate. As a solvent used for the graphene oxide dispersion liquid, apolar solvent is preferably used, and water, NMP, or the like can beused.

FIG. 5A is a schematic view of a spray dry apparatus 280. The spray dryapparatus 280 includes a chamber 281 and a nozzle 282. Dispersion liquid284 is supplied to the nozzle 282 through a tube 283. The dispersionliquid 284 is supplied from the nozzle 282 to the chamber 281 in theform of mist and dried in the chamber 281.

The nozzle 282 may be heated with a heater 285. Here, a region of thechamber 281 which is close to the nozzle 282, for example, a regionsurrounded by dashed-two dotted line in FIG. 5A, is also heated with theheater 285. In the case of using graphene oxide dispersion liquid as thedispersion liquid 284, part of graphene oxide supplied from the grapheneoxide dispersion liquid is collected as powder to a collection container286 through the chamber 281, and another part of the graphene oxide isdeposited as the graphene compound sheet 222 on a wall surface of thechamber 281. The air in the chamber 281 may be suctioned by an aspiratoror the like through a path indicated by an arrow 288.

Alternatively, a substrate may be set in the chamber 281 and a graphenecompound sheet may be deposited over the substrate. The substrate mayhave a flat-plate like shape or a curved surface. The substrate may beset parallel to the nozzle 282 or at a certain angle. For example, thesubstrate may be set perpendicular to the nozzle 282. FIG. 5Billustrates an example in which a substrate 287 is set perpendicular tothe nozzle 282 and the graphene compound sheet 222 is deposited over thesubstrate. Here, the thickness uniformity in the sheet can be improvedin some cases by deposition while the nozzle 282 is moved from side toside, for example. Alternatively, the substrate 287 may be moved fromside to side. Further alternatively, both the nozzle 282 and thesubstrate 287 may be moved from side to side.

The formed graphene compound sheet 222 preferably has a peak in a rangeof greater than or equal to 7° and less than or equal to 10° in XRDevaluation, for example. Here, a “peak” means a maximum value and/ormaximum value, for example.

Here, the interlayer distance in the graphene compound sheet 222 isgreater than 0.8 nm and less than or equal to 2 nm, or greater than orequal to 0.85 nm and less than or equal to 1.3 nm.

In observation with a TEM, a small region, e.g.: a several-nanometer toseveral-micrometer square region, is observed. In XRD evaluation,average data on a larger region can be evaluated, for example.

The interlayer distance observed by TEM is smaller than that calculatedfrom XRD evaluation in some cases. For example, the interlayer distancein the graphene compound sheet 222 which is observed by TEM is smallerthan 0.5 nm in some cases.

Next, the graphene compound sheet 222 is subjected to reductiontreatment to obtain the conductor 201. Here, the conductor 201 can bereferred to as a graphene compound sheet. When the graphene compoundsheet 222 is subjected to reduction treatment, graphene oxide includedin the graphene compound sheet 222 is reduced, leading to higherconductivity. Accordingly, the conductor 201 has higher conductivitythan the graphene compound sheet 222. Through the above process, thesheet-like conductor 201 is obtained. For example, the sheet-likeconductor 201 may be processed by being cut so that the belt-shapedconductor 201 is obtained.

As a method for reduction treatment, chemical reduction in whichreduction is performed with reaction with a reducing agent, thermalreduction in which heat treatment is performed, or the like can be used.

The chemical reduction are described. Examples of the reducing agentinclude ascorbic acid, hydrazine, dimethyl hydrazine, hydroquinone,sodium boron hydride (NaBH₄), lithium aluminum hydride (LiAlH₄),N,N-diethylhydroxylamine, and a derivative thereof. For example,ascorbic acid and hydroquinone are preferable to hydrazine and sodiumtetrahydroborate in that they are safe owing to low reducing ability andutilized industrially with ease.

A polar solvent can be used as the solvent. Any material can be used forthe polar solvent as long as it can dissolve the reducing agent.Examples of the material of the solvent include water, methanol,ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF),1-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), ethyleneglycol, diethylene glycol, glycerin, and a mixed solution of any two ormore of the above.

As the reducing solution containing a reducing agent and a solvent, amixed solution of ethanol and ascorbic acid, or a mixed solution ofwater, ascorbic acid, and lithium hydroxide can be used.

Protons are added to graphene oxide by ascorbic acid. Then, the grapheneoxide is reduced by release of H₂O.

The reduction temperature in the chemical reduction is, for example,higher than or equal to room temperature and lower than or equal to 100°C., preferably higher than or equal to 40° C. and lower than or equal to70° C. The treatment time can be longer than or equal to 3 minutes andshorter than or equal to 10 hours.

After the reduction treatment, cleaning may be performed. The washing ispreferably performed using a solution given as the solvent contained inthe reducing solution. The solution may be either the same as ordifferent from the solvent contained in the reducing solution. After thecleaning, drying may be performed.

Next, thermal reduction is described. The step for thermal reduction isperformed, for example, at a temperature higher than or equal to 50° C.and lower than 500° C., preferably higher than or equal to 120° C. andlower than or equal to 400° C. for 1 hour to 48 hours. The step forthermal reduction may be performed under a reduced pressure (in vacuum),in a reduction atmosphere, or under an atmospheric pressure. As a gas,air or an inert gas such as nitrogen or another gas may be used.

Here, the conductor 201 obtained after reduction preferably has a peakin a range of greater than or equal to 21° and less than or equal to 27°in XRD evaluation, for example.

Embodiment 2

In this embodiment, an example in which the conductor 201 of oneembodiment of the present invention is applied to an electrode isdescribed. The conductor 201 includes the graphene compound 211.

The conductor 201 of one embodiment of the present invention preferablyserves as an active material of an electrode. That is, the conductor 201preferably contributes to a charging reaction or a discharging reaction.When the conductor 201 of one embodiment of the present invention isused as an electrode of a power storage device, for example, theconductor 201 may contribute to an oxidation-reduction reaction. Here,oxidation-reduction reaction means donation and acceptance of electrons,for example.

As a more specific example, an oxidation-reduction reaction meansdonation and acceptance of electrons by a reaction with an ion such asan anion or a cation.

Alternatively, the conductor 201 of one embodiment of the presentinvention can be used as an electrode of a capacitor in some cases by anelectric double layer being formed over its surface. The surface area ofthe conductor of one embodiment of the present invention is larger thanthat of graphite in some cases, for example. When an electrode with alarge surface area is used, the capacity of a power storage device canbe increased.

The conductor 201 may also be used as a current collector of anelectrode. For example, an electrode of one embodiment of the presentinvention may include the conductor 201 and an active material otherthan the conductor 201. When the conductor 201 is used as a currentcollector, the current collector can be thinned, leading to a reductionin weight of the electrode in some cases. Furthermore, the electrode maybe likely to be changed in its shape.

An example of an electrode is described below.

Example 1 of Electrode

The inventors have found that the power storage device can have highcapacity in the case where the conductor 201 of one embodiment of thepresent invention is used as an electrode of the power storage device.Although the detail is described in Examples to be described later, itwas found that in the case where the conductor 201 was used as anelectrode, the discharge capacity was as high as about 100 mAh/g in somecases. Here, the discharge capacity of the conductor 201 is preferablyhigher than or equal to 10 mAh/g, and further preferably higher than orequal to 40 mAh/g, for example.

An example in which the conductor 201 of one embodiment of the presentinvention is used as an electrode of a power storage device isdescribed. The conductor 201 includes the graphene compound 211. Theabove embodiment can be referred to for the graphene compound 211.

A power storage device 100 illustrated in FIG. 6A includes the conductor201, a separator 107, and an electrode 151. The separator 107 isinterposed between the conductor 201 and the electrode 151. The powerstorage device 100 preferably includes an electrolyte solution (notillustrated) between the conductor 201 and the electrode 151. Asillustrated in FIG. 6B, the power storage device 100 may include aplurality of pairs each including the electrode 151 and the conductor201 between which the separator 107 is interposed. For example, theelectrode 151 includes a current collector and a first layer over thecurrent collector. Here, the first layer preferably includes an activematerial. The components of the electrode 151 are described in detaillater.

The power storage device 100 illustrated in FIG. 7A includes theconductor 201, an electrolyte 158, and the electrode 151. Theelectrolyte 158 is interposed between the conductor 201 and theelectrode 151. As the electrolyte 158, a solid electrolyte is preferablyused, for example. The electrolyte 158 is in contact with both theconductor 201 and the electrode 151. As illustrated in FIG. 7B, thepower storage device 100 may include a plurality of pairs each includingthe electrode 151 and the conductor 201 between which the electrolyte158 is interposed.

Here, the graphene compound sheet used as the conductor 201 can serve asboth a current collector and an active material. Accordingly, thecapacity of the power storage device can be increased as compared to thecase where an active material is provided over a current collector ofmetal foil or the like.

<Method for Forming Conductor>

Here, for example, a sheet-like separator or a belt-like separator maybe used as the separator 107, and the conductor 201 may be formed overat least one of surfaces of the separator 107. The conductor 201 can beformed by a spray dry method, a coating method, or the like. A coatingmethod is described later.

As the electrolyte solution and the separator, an electrolyte solutionand a separator which are described in detail in an embodiment below canbe used.

Example 2 of Electrode

Next, an example of an electrode including the conductor 201 of oneembodiment of the present invention and an active material differentfrom the conductor 201 is described. Electrodes 101 illustrated in FIGS.8A and 8B each include the conductor 201 and a layer 102. The layer 102includes an active material 103. The layer 102 is preferably providedover at least one of surfaces of the conductor 201. In FIG. 8A, onesurface of the conductor 201 is provided with the layer 102. In FIG. 8B,both surfaces of the conductor 201 are provided with the layers 102.

The power storage device 100 illustrated in FIG. 9A includes theelectrode 101, the electrode 151, and the separator 107. As illustratedin FIG. 9B, the power storage device 100 may include a plurality ofpairs each including the electrode 101 and the electrode 151 betweenwhich the separator 107 is interposed. It is preferable that the powerstorage devices 100 illustrated in FIGS. 9A and 9B each include anelectrolyte solution (not illustrated) between the electrode 101 and theelectrode 151. The power storage devices 100 may each include theelectrolyte 158 between the electrode 101 and the electrode 151.

The layer 102 may include a conductive additive, a binder, or the like.

In the case where the electrode 101 is a positive electrode, the layer102 preferably include a positive electrode active material as theactive material 103. In the case where the electrode 101 is a negativeelectrode, the layer 102 preferably include a negative electrode activematerial as the active material 103.

In the case where the electrode 101 is a positive electrode, theelectrode 151 preferably includes a negative electrode active materialas the active material, and in the case where the electrode 101 is anegative electrode, the electrode 151 preferably includes a positiveelectrode active material as the active material.

For example, the electrode 151 includes a current collector and a firstlayer over the current collector. Here, the first layer preferablyincludes an active material. The first layer may include a conductiveadditive, a binder, or the like.

The current collector included in the electrode 151 can be formed usinga material that has high conductivity, such as a metal like stainlesssteel, gold, platinum, aluminum, or titanium, or an alloy thereof. Inthe case where the current collector is used in the positive electrode,it is preferred that it not dissolve at the potential of the positiveelectrode. In the case where the current collector is used in thenegative electrode, it is preferred that it not be alloyed with carrierions such as lithium ions. Alternatively, an aluminum alloy to which anelement which improves heat resistance, such as silicon, titanium,neodymium, scandium, or molybdenum, is added can be used. Stillalternatively, a metal element which forms silicide by reacting withsilicon can be used. Examples of the metal element which forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, andthe like. The current collector can have a foil-like shape, a plate-likeshape (sheet-like shape), a net-like shape, a punching-metal shape, anexpanded-metal shape, or the like as appropriate. The current collectorpreferably has a thickness of more than or equal to 5 μm and less thanor equal to 30 μm. The conductor 201 may be used as the currentcollector included in the electrode 151.

Examples of a positive electrode active material include a compositeoxide with an olivine crystal structure, a composite oxide with alayered rock-salt crystal structure, and a composite oxide with a spinelcrystal structure.

As the positive electrode active material, a compound such as LiFeO₂,LiCoO₂, LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, or MnO₂ can be used. LiCoO₂ isparticularly preferable because it has high capacity, stability in theair higher than that of LiNiO₂, and thermal stability higher than thatof LiNiO₂, for example. It is preferable to add a small amount oflithium nickel oxide (LiNiO₂ or LiNi_(1-x)M_(x)O₂ (M=Co, Al, or thelike)) to a lithium-containing material with a spinel crystal structurewhich contains manganese such as LiMn₂O₄ because characteristics of thesecondary battery using such a material can be improved.

The average diameter of primary particles of the positive electrodeactive material is preferably greater than or equal to 5 nm and lessthan or equal to 50 m, further preferably greater than or equal to 100nm and less than or equal to 500 nm, for example. Furthermore, thespecific surface area is preferably greater than or equal to 5 m²/g andless than or equal to 15 m²/g. Furthermore, the average size ofsecondary particles is preferably greater than or equal to 5 m and lessthan or equal to 50 m. Note that the average particle sizes can bemeasured with a particle size distribution analyzer or the like using alaser diffraction and scattering method or by observation with ascanning electron microscope (SEM) or a TEM. The specific surface areacan be measured by a gas adsorption method.

In addition, a lithium-manganese composite oxide that is represented bya composition formula Li_(a)Mn_(b)M_(c)O_(d) can be used as the positiveelectrode active material.

Here, the element M is preferably a metal element other than lithium andmanganese, or silicon or phosphorus, further preferably nickel.Furthermore, in the case where a whole particle of a lithium manganesecomplex oxide is measured, it is preferable to satisfy the following atthe time of discharging: 0<a/(b+c)<2; c>0; and 0.26≤(b+c)/d<0.5. Toachieve high capacity, the lithium manganese complex oxide preferablyincludes a region where the surface portion and the middle portion aredifferent in the crystal structure, the crystal orientation, or theoxygen content. In order that such a lithium-manganese composite oxidecan be obtained, the composition formula is preferablyLi_(a)Mn_(b)Ni_(c)O_(d) satisfying the following: 1.6≤a≤1.848;0.19≤c/b≤0.935; and 2.5≤d≤3.

Furthermore, it is particularly preferable to use a lithium-manganesecomposite oxide represented by a composition formulaLi_(1.68)Mn_(0.8062)Ni_(0.318)O₃. In this specification and the like, alithium-manganese composite oxide represented by a composition formulaLi_(1.68)Mn_(0.062)Ni_(0.318)O₃ refers to that formed at a ratio (molarratio) of the amounts of raw materials ofLi₂CO₃:MnCO₃:NiO=0.84:0.8062:0.318. Although this lithium-manganesecomposite oxide is represented by a composition formulaLi_(1.68)Mn_(0.8062)Ni_(0.318)O₃, the composition might deviate fromthis.

Note that the ratios of metal, silicon, phosphorus, and other elementsto the total composition in the whole particle of a lithium manganesecomplex oxide can be measured with, for example, an inductively coupledplasma mass spectrometer (ICP-MS). The ratio of oxygen to the totalcomposition in the whole particle of a lithium manganese complex oxidecan be measured by, for example, energy dispersive X-ray spectroscopy(EDX). Alternatively, the composition ratio of oxygen in the wholeparticle of a lithium-manganese composite oxide can be measured byICP-MS combined with fusion gas analysis and valence evaluation of X-rayabsorption fine structure (XAFS) analysis. Note that thelithium-manganese composite oxide is an oxide containing at leastlithium and manganese, and may contain at least one selected fromchromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc,indium, gallium, copper, titanium, niobium, silicon, phosphorus, and thelike.

FIGS. 10A and 10B each illustrate an example of a cross-sectional viewof a particle of a lithium-manganese composite oxide having a regionwith different crystal structures, crystal orientations, or oxygencontents.

As illustrated in FIG. 10A, the lithium-manganese composite oxide havinga region with different crystal structures, crystal orientations, oroxygen contents preferably include a region 331, a region 332, and aregion 333. The region 332 is in contact with at least part of the outerside of the region 331. Here, the term “outer side” refers to the sidecloser to a surface of a particle. The region 333 preferably includes aregion corresponding to a surface of a particle containing thelithium-manganese composite oxide.

As shown in FIG. 10B, the region 331 may include a region not coveredwith the region 332. The region 332 may include a region not coveredwith the region 333. Furthermore, the region 331 may include a region incontact with the region 333, for example. Furthermore, the region 331may include a region covered with neither the region 332 nor the region333.

The region 332 preferably has composition different from that of theregion 331.

For example, the case will be described where the composition of theregion 331 and that of the region 332 are separately measured and theregion 331 and the region 332 each contain lithium, manganese, theelement M, and oxygen; the atomic ratio of lithium to manganese, theelement M, and oxygen in the region 331 is represented by a1:b1:c1:d1;and the atomic ratio of lithium to manganese, the element M, and oxygenin the region 332 is represented by a2:b2:c2:d2. Note that thecomposition of each of the region 331 and the region 332 can be measuredby energy dispersive X-ray spectroscopy (EDX) using a transmissionelectron microscope (TEM), for example. In measurement by EDX, theproportion of lithium is sometimes difficult to measure. Thus, adifference between the region 331 and the region 332 in compositionexcept for lithium will be described below. Here, d1/(b1+c1) ispreferably greater than or equal to 2.2, further preferably greater thanor equal to 2.3, and still further preferably greater than or equal to2.35 and less than or equal to 3. Furthermore, d2/(b2+c2) is preferablyless than 2.2, further preferably less than 2.1, and still furtherpreferably greater than or equal to 1.1 and less than or equal to 1.9.In this case, the composition of the whole particle of lithium-manganesecomposite oxide including the region 331 and the region 332 alsopreferably satisfies the above inequality: 0.26≤(b+c)/d<0.5.

The valence of manganese in the region 332 may be different from that ofmanganese in the region 331. The valence of the element Min the region332 may be different from that of the element Min the region 331.

Specifically, the region 331 is preferably a lithium-manganese compositeoxide having a layered rock-salt crystal structure. The region 332 ispreferably a lithium-manganese composite oxide having a spinel crystalstructure.

Here, in the case where the composition of the regions or valences ofelements in the regions are spatially distributed, the composition orvalences in a plurality of portions are obtained, the average valuesthereof are calculated, and the average values are regarded as thecomposition or valences of the regions, for example.

A transition layer may be provided between the region 332 and the region331. The transition layer is a region where the composition, crystalstructure, or crystal lattice constant changes continuously orgradually. A mixed layer may be provided between the second region andthe first region. The mixed layer is a layer in which, for example, twoor more crystals having different crystal orientations are mixed, two ormore crystals having different crystal structures are mixed, or two ormore crystals having different compositions are mixed.

The region 333 preferably contains carbon or a metal compound. Examplesof the metal include cobalt, aluminum, nickel, iron, manganese,titanium, zinc, and lithium. The region 333 may contain an oxide or afluoride of the metal as an example of the metal compound.

It is particularly preferable that the region 333 contain carbon. Sincecarbon has high conductivity, a particle coated with carbon in anelectrode of the power storage device can reduce the resistance of theelectrode, for example. The region 333 preferably includes a graphenecompound. The use of the graphene compound in the region 333 allows thelithium-manganese composite oxide particle to be efficiently coated withthe region 333. The graphene compound will be described later. Theregion 333 may include, specifically, graphene or graphene oxide, forexample. Furthermore, graphene formed by reducing graphene oxide ispreferably used as graphene. Graphene has excellent electricalcharacteristics of high conductivity and excellent physical propertiesof high flexibility and high mechanical strength. When graphene oxide isused for the region 333 and is reduced, the region 332 in contact withthe region 333 is oxidized in some cases.

When the region 333 includes a graphene compound, the secondary batteryusing the lithium-manganese composite oxide as a positive electrodematerial can have improved cycle performance.

The thickness of the region 333 is preferably greater than or equal to0.4 nm and less than or equal to 40 nm.

Furthermore, the average size of primary particles of thelithium-manganese composite oxide is preferably greater than or equal to5 nm and less than or equal to 50 m, and further preferably greater thanor equal to 100 nm and less than or equal to 500 nm, for example.Furthermore, the specific surface area is preferably greater than orequal to 5 m²/g and less than or equal to 15 m²/g. Furthermore, theaverage size of secondary particles is preferably greater than or equalto 5 m and less than or equal to 50 m.

As a negative electrode active material, for example, a carbon-basedmaterial or an alloy-based material can be used.

As the carbon-based material, graphite, graphitizing carbon (softcarbon), non-graphitizing carbon (hard carbon), a carbon nanotube,graphene, carbon black, or the like can be used.

Examples of graphite include artificial graphite and natural graphite.Examples of artificial graphite include meso-carbon microbeads (MCMB),coke-based artificial graphite, and pitch-based artificial graphite. Asartificial graphite, spherical graphite having a spherical shape can beused. For example, MCMB is preferably used because it may have aspherical shape. Moreover, MCMB may preferably be used because it canrelatively easily have a small surface area. Examples of naturalgraphite include flake graphite and spherical natural graphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.1 V and lower than or equal to 0.3 Vvs. Li/Li⁺) when lithium ions are intercalated into the graphite (whilea lithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage. Inaddition, graphite is preferred because of its advantages such as arelatively high capacity per unit volume, relatively small volumeexpansion, low cost, and higher level of safety than that of a lithiummetal.

For the negative electrode active material, an element which enablescharge-discharge reactions by an alloying reaction and a dealloyingreaction with lithium can be used. For example, a material containing atleast one of silicon, tin, gallium, aluminum, germanium, lead, antimony,bismuth, silver, zinc, cadmium, indium, and the like can be used. Suchelements have higher capacity than carbon. In particular, silicon has ahigh theoretical capacity of 4200 mAh/g. For this reason, silicon ispreferably used as the negative electrode active material.Alternatively, a compound containing any of the above elements may beused. Examples of the compound include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like. Here, anelement that enables charge-discharge reactions by an alloying reactionand a dealloying reaction with lithium, a compound containing theelement, and the like may be referred to as an alloy-based material.

In this specification and the like, SiO refers, for example, to siliconmonoxide. SiO can alternatively be expressed as SiOx. Here, x preferablyhas an approximate value of 1. For example, x is preferably 0.2 or moreand 1.5 or less, and further preferably 0.3 or more and 1.2 or less.

Alternatively, as the negative electrode active material, an oxide suchas titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (LixC₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, as the negative electrode active material,L_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material and thus the negative electrode active material can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedas the negative electrode active material; for example, a transitionmetal oxide which does not form an alloy with lithium, such as cobaltoxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used.Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂₀₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃. Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, or CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

The reaction potential of the negative electrode active material ispreferably as low as possible, in which case the voltage of the powerstorage device can be high. On the other hand, when the potential islow, power of reducing an electrolyte solution is increased, so that anorganic solvent or the like in an electrolyte solution might besubjected to reductive decomposition. The range of potentials in whichthe electrolysis of an electrolyte solution does not occur is referredto as a potential window. The electrode potential of the negativeelectrode needs to be within a potential window of an electrolytesolution; however, the potentials of many active materials used fornegative electrodes of lithium-ion secondary batteries and lithium-ioncapacitors are out of the potential windows of almost all electrolytesolutions. Specifically, materials with low reaction potentials such asgraphite and silicon can increase the voltage of power storage devicesbut are likely to cause the reductive decomposition of electrolytesolutions.

Carrier ions such as lithium ions may be occluded by a negativeelectrode active material in advance.

The electrode 101 and the electrode 151 may each include a conductiveadditive. Examples of the conductive additive include a carbon material,a metal material, and a conductive ceramic material. Alternatively, afiber material may be used as the conductive additive. The content ofthe conductive additive in the active material layer is preferablygreater than or equal to 1 wt % and less than or equal to 10 wt %, andfurther preferably greater than or equal to 1 wt % and less than orequal to 5 wt %.

A network for electrical conduction can be formed in the electrode bythe conductive additive. The conductive additive also allows maintainingof a path for electric conduction between the particles of the positiveelectrode active material. The addition of the conductive additive tothe active material layer increases the electrical conductivity of theactive material layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (e.g., acetylene black (AB)), graphite (black lead)particles, graphene, and fullerene. Alternatively, metal powder or metalfibers of copper, nickel, aluminum, silver, gold, or the like, aconductive ceramic material, or the like can be used.

As the conductive additive, a graphene compound may be used.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength in some cases. Furthermore, a graphene compoundhas a planar shape. A graphene compound is capable of makinglow-resistance surface contact and has extremely high conductivity evenwith a small thickness in some cases. Therefore, even with a smallamount of a graphene compound, a conductive path can be formedefficiently in an active material layer. For this reason, it ispreferable to use a graphene compound as the conductive additive becausethe area where the active material and the conductive additive are incontact with each other can be increased. In addition, it is preferableto use a graphene compound as the conductive additive because theelectrical resistance can be reduced in some cases. Here, it isparticularly preferable to use, for example, graphene, multilayergraphene, or RGO as a graphene compound.

In the case where an active material with a small average particle size(e.g., 1 m or less) is used, the specific surface area of the activematerial is large and thus more conductive paths for the activematerials are needed. In such a case, it is particularly preferable touse a graphene compound that can efficiently form a conductive path evenwith a small amount.

A cross-sectional structure example of the layer 102 including agraphene compound as a conductive additive is described below.

FIG. 11A is a longitudinal sectional view of the layer 102. The layer102 includes particles of the active material 103, a graphene compound321 serving as a conductive additive, and a binder 104. Here, grapheneor multilayer graphene may be used as the graphene compound 321, forexample. The graphene compound 321 preferably has a sheet-like shape.The graphene compound 321 may have a sheet-like shape formed of aplurality of sheets of multilayer graphene and/or a plurality sheets ofgraphene that partly overlap with each other. FIG. 11B is an enlargedview of a region surrounded by dashed-dotted line in FIG. 11A.

The longitudinal section of the layer 102 of FIG. 11A showssubstantially uniform dispersion of the graphene compounds 321 in thelayer 102. The graphene compounds 321 are schematically shown by thicklines in FIG. 11A but are actually thin films each having a thicknesscorresponding to the thickness of a single layer or a multilayer ofcarbon molecules. The plurality of graphene compounds 321 are formed insuch a way as to wrap, coat, or adhere to the surfaces of the pluralityof particles of the active material 103, so that the graphene compounds321 make surface contact with the plurality of particles of the activematerial 103.

Here, the plurality of graphene compounds are bonded to each other toform a net-like graphene compound sheet (hereinafter referred to as agraphene compound net or a graphene net). The graphene net covering theactive material can function as a binder for bonding active materials.The amount of a binder can thus be reduced, or the binder does not haveto be used. This can increase the proportion of the active material inthe electrode volume or weight. That is, the capacity of the powerstorage device can be increased.

Here, it is preferable to perform reduction after a layer to be thelayer 102 is formed in such a manner that graphene oxide is used as thegraphene compound 321 and mixed with an active material. When grapheneoxide with extremely high dispersibility in a polar solvent is used forthe formation of the graphene compounds 321, the graphene compounds 321can be substantially uniformly dispersed in the layer 102. The solventis removed by volatilization from a dispersion medium in which grapheneoxide is uniformly dispersed, and the graphene oxide is reduced; hence,the graphene compounds 321 remaining in the layer 102 partly overlapwith each other and are dispersed such that surface contact is made,thereby forming a three-dimensional conduction path. Note that grapheneoxide can be reduced either by heat treatment or with the use of areducing agent, for example.

Unlike a conductive additive in the form of particles, such as acetyleneblack, which makes point contact with an active material, the graphenecompound 321 is capable of making low-resistance surface contact;accordingly, the electrical conduction between the particles of theactive material 103 and the graphene compounds 321 can be improved witha smaller amount of the graphene compound 321 than that of a normalconductive additive. Thus, the proportion of the particles of the activematerial 103 in the layer 102 can be increased. Accordingly, thedischarge capacity of the power storage device can be increased.

The electrode 101 and the electrode 151 may each include a binder. Asthe binder, for example, a rubber material such as styrene-butadienerubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadienerubber, butadiene rubber, or ethylene-propylene-diene copolymer can beused. Alternatively, fluororubber can be used as the binder.

For the binder, for example, water-soluble polymers are preferably used.As the water-soluble polymers, a polysaccharide or the like can be used.As the polysaccharide, a cellulose derivative such as carboxymethylcellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropylcellulose, diacetyl cellulose, or regenerated cellulose, starch, or thelike can be used. It is more preferred that such water-soluble polymersbe used in combination with any of the above rubber materials.

Alternatively, as the binder, a material such as polystyrene,poly(methyl acrylate), polymethylmethacrylate (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, isobutylene,polyethylene terephthalate, nylon, polyvinylidene fluoride (PVdF),polyacrylonitrile (PAN), polyvinyl chloride, ethylene-propylene-dienepolymer, polyvinyl acetate, or nitrocellulose is preferably used.

Two or more of the above materials may be used in combination for thebinder.

For example, a material having a significant viscosity modifying effectand another material may be used in combination. For example, a rubbermaterial or the like has high adhesion or high elasticity but may havedifficulty in viscosity modification when mixed in a solvent. In such acase, for example, it is preferable to mix with a material having asignificant viscosity modifying effect. As a material having asignificant viscosity modifying effect, for example, a water-solublepolymer is preferably used. An example of a water-soluble polymer havingan especially significant viscosity modifying effect is theabove-mentioned polysaccharide; for example, a cellulose derivative suchas carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose,or starch can be used.

Note that a cellulose derivative such as carboxymethyl cellulose obtainsa higher solubility when converted into a salt such as a sodium salt oran ammonium salt of carboxymethyl cellulose, and accordingly, easilyexerts an effect as a viscosity modifier. The high solubility can alsoincrease the dispersibility of an active material and other componentsin the formation of slurry for an electrode. In this specification,cellulose and a cellulose derivative used as a binder of an electrodeinclude salts thereof.

The water-soluble polymers stabilize viscosity by being dissolved inwater and allow stable dispersion of the active material andstyrene-butadiene rubber in an aqueous solution. Furthermore, awater-soluble polymer is expected to be easily and stably adsorbed to anactive material surface because it has a functional group. Manycellulose derivatives such as carboxymethyl cellulose have functionalgroups such as a hydroxyl group and a carboxyl group. Because offunctional groups, polymers are expected to interact with each other andcover an active material surface in a large area.

In the case where the binder covering or being in contact with theactive material surface forms a film, the film is expected to serve as apassivation film to suppress the decomposition of the electrolytesolution. Here, the passivation film refers to a film without electricconductivity or a film with extremely low electric conductivity, and caninhibit the decomposition of an electrolyte solution at a potential atwhich a battery reaction occurs in the case where the passivation filmis formed on the active material surface, for example. It is preferredthat the passivation film can conduct lithium ions while suppressingelectric conduction.

<Method for Forming Electrode>

An example of a method for forming an electrode is described. First, theconductor 201 is prepared. Here, the sheet-like conductor 201 is used asan example. The above embodiment can be referred to for the method forforming the conductor 201.

Next, the active material 103 and a solvent are mixed to form a firstmixture. Here, a conductive additive or a binder may be added and mixed.A polar solvent can be used as the solvent. Examples of the material ofthe polar solvent are water, methanol, ethanol, acetone, tetrahydrofuran(THF), dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO), and a mixed solution of any two or more of the above.In the case where a rubber material and a water-soluble polymer are usedas the first material and the second material, respectively, water ispreferably used as the solvent.

Next, the formed first mixture is applied to the conductor 201. Afterthat, the solvent in the first mixture is volatilized by heat treatmentor the like, so that the layer 102 is formed.

Embodiment 3

In this embodiment, power storage devices of embodiments of the presentinvention are described.

Examples of the power storage device of one embodiment of the presentinvention include a secondary battery such as a lithium ion batterywhich utilizes an electrochemical reaction; an electrochemical capacitorsuch as an electric double-layer capacitor or a redox capacitor; an airbattery; and a fuel battery.

<Wound Storage Battery>

As an example of the power storage device, a wound storage battery isdescribed. FIG. 12 illustrates an example of the wound storage battery.

A power storage device 980 illustrated in FIG. 12 includes a wound body993 and an exterior body 983. The wound body 993 includes the electrode101, the electrode 151, and a separator 996. A space surrounded by theexterior body 983 is filled with an electrolyte solution. The powerstorage device 980 preferably includes a lead electrode 997 connected tothe conductor 201 and a lead electrode 998 connected to the electrode151. The above embodiment can be referred to for the electrode 101 andthe electrode 151.

Here is described an example of using, as the electrode 101, anelectrode including the layer 102 formed over the belt-like conductor201. Here, an example in which the layer 102 includes alithium-manganese composite oxide as an active material is described.

The area of the conductor 201 may be, for example, greater than or equalto 1 mm² and less than or equal to 100 mm², greater than or equal to 25mm² and less than or equal to 10 m², or greater than or equal to 100 mm²and less than or equal to 3 m².

Note that although an example in which the electrode including the layer102 formed over the belt-like conductor 201 is used as the electrode 101is described here, an electrode which does not include the layer 102 canalso be used in the power storage device 980 illustrated in FIG. 12.

First, the electrode 101 and the electrode 151 are prepared. Thebelt-like electrode 151 may be formed in such a manner that an activematerial layer is formed over at least one of surfaces of a belt-likecurrent collector. Here, as an example, copper is used for the currentcollector, and graphite is used as an active material included in theactive material layer. Next, as illustrated in FIG. 13A, the leadelectrode 997 is bonded to the conductor 201 included in the electrode101, and the lead electrode 998 is bonded to the current collectorincluded in the electrode 151. The bonding of the lead electrodes can beperformed by ultrasonic welding, for example.

Next, as illustrated in FIG. 13B, the electrode 101, the separator 996,the electrode 151, and the separator 996 are stacked in this order fromthe bottom. After that, as illustrated in FIG. 14A, the electrode 101,the electrode 151, and the separators 996, which are stacked, are woundto form the wound body 993. At this time, they may be wound with a core.

Next, as illustrated in FIG. 14B, the wound body 993 is surrounded by anexterior body 983 a and an exterior body 983 b. After that, the exteriorbody 983 a and the exterior body 983 b are bonded to each other. Here,the bonded exterior body is referred to as an exterior body 983. It ispreferable to leave a portion of the exterior body 983 where theexterior body 983 a and the exterior body 983 b are not bonded to eachother for introduction of an electrolyte solution. The unbonded portionis referred to as an inlet in some cases. An electrolyte solution isinjected from the inlet. After that, the inlet is bonded, so that thepower storage device 980 is manufactured.

Here, the conductor 201 may be formed over one of surfaces of thebelt-like separator 996, and then, the layer 102 may be formed over theconductor 201. That is, the structure may be as follows: the conductor201 and the layer 102 are stacked over the separator 996, the separator996, the conductor 201, and the layer 102 are fixed, and then, theelectrode 151 and the separator 996 are stacked.

In the case where an aprotic organic solvent is used as a solvent of theelectrolyte solution, for example, one of ethylene carbonate (EC),propylene carbonate (PC), butylene carbonate, chloroethylene carbonate,vinylene carbonate (VC), g-butyrolactone, g-valerolactone, dimethylcarbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC),methyl formate, methyl acetate, methyl butyrate, 1,3-dioxane,1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether,methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane,and sultone can be used, or two or more of these solvents can be used inan appropriate combination in an appropriate ratio.

When a gelled high-molecular material is used as the solvent of theelectrolyte solution, safety against liquid leakage and the like isimproved. Furthermore, a secondary battery can be thinner and morelightweight. Typical examples of the high-molecular material thatundergoes gelation include a silicone gel, an acrylic gel, anacrylonitrile gel, a polyethylene oxide-based gel, a polypropyleneoxide-based gel, a fluorine-based polymer gel, and the like.

Alternatively, the use of one or more kinds of ionic liquids (roomtemperature molten salts) which have features of non-flammability andnon-volatility as a solvent of the electrolyte solution can prevent apower storage device from exploding or catching fire even when a powerstorage device internally shorts out or the internal temperatureincreases owing to overcharging or the like. An ionic liquid contains acation and an anion. The ionic liquid of one embodiment of the presentinvention contains an organic cation and an anion. Examples of theorganic cation used for the electrolyte solution include aliphatic oniumcations such as a quaternary ammonium cation, a tertiary sulfoniumcation, and a quaternary phosphonium cation, and aromatic cations suchas an imidazolium cation and a pyridinium cation. Examples of the anionused for the electrolyte solution include a monovalent amide-basedanion, a monovalent methide-based anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, a tetrafluoroborate anion, aperfluoroalkylborate anion, a hexafluorophosphate anion, and aperfluoroalkylphosphateanion.

In the case of using lithium ions as carriers, as a salt used for theelectrolyte solution, one of lithium salts such as LiPF₆, LiClO₄,LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀,Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃,LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂) (CF₃SO₂), and LiN(C₂F₅SO₂)₂ can be used, ortwo or more of these lithium salts can be used in an appropriatecombination in an appropriate ratio.

The electrolyte solution used for the power storage device is preferablya highly purified one so as to contain a negligible amount of dustparticles and elements other than the constituent elements of theelectrolyte solution (hereinafter, also simply referred to asimpurities). Specifically, the weight ratio of impurities to theelectrolyte solution is less than or equal to 1%, preferably less thanor equal to 0.1%, and more preferably less than or equal to 0.01%.

Furthermore, an additive agent such as vinylene carbonate, propanesultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC),or LiBOB may be added to the electrolyte solution. The concentration ofsuch an additive agent in the whole solvent is, for example, higher thanor equal to 0.1 wt % and lower than or equal to 5 wt %.

Alternatively, a polymer gel electrolyte obtained in such a manner thata polymer is swelled with an electrolyte solution may be used.

Examples of the polymer include a polymer having a polyalkylene oxidestructure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile;and a copolymer containing any of them. For example, PVDF-HFP, which isa copolymer of PVDF and hexafluoropropylene (HFP) can be used. Theformed polymer may be porous.

Instead of the electrolyte solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including amacromolecular material such as a polyethylene oxide (PEO)-basedmacromolecular material may alternatively be used. In the case of usingthe solid electrolyte, a separator or a spacer is not necessary.Furthermore, the battery can be entirely solidified; therefore, there isno possibility of liquid leakage and thus the safety of the battery isdramatically increased.

As the separator 996, paper; nonwoven fabric; glass fiber; ceramics;synthetic fiber containing nylon (polyamide), vinylon (polyvinylalcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane;or the like can be used.

A storage battery 990 illustrated in FIG. 15A includes an exterior body991 and an exterior body 992 which have a shape different from theexterior body in FIG. 12. FIG. 15B illustrates components of the storagebattery 990. The above wound body 993 is stored in the exterior body991.

For example, a metal material such as aluminum or a resin material canbe used for the exterior bodies 983, 991, and 992. With the use of aresin material for the exterior bodies, the exterior bodies can bedeformed when external force is applied; thus, a flexible storagebattery can be manufactured. As the exterior body, for example, a filmhaving a three-layer structure in which a highly flexible metal thinfilm of aluminum, stainless steel, copper, nickel, or the like isprovided over a film formed of a material such as polyethylene,polypropylene, polycarbonate, ionomer, or polyamide, and an insulatingsynthetic resin film of a polyamide-based resin, a polyester-basedresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used.

Here, for example, in the case where the electrode 101 is used as apositive electrode, the electrode 151 serves as a negative electrode,and in the case where the electrode 101 is used as a negative electrode,the electrode 151 serves as a positive electrode.

Specifically, for example, the electrode 101 is used as a positiveelectrode, any of the positive electrode active materials described inEmbodiment 2 is used as the active material 103 included in theelectrode 101. For example, a lithium-manganese composite oxiderepresented by Li_(a)Mn_(b)MO_(a) is used as the active material 103.The electrode 151 is used as a negative electrode. The electrode 151includes any of the negative electrode active materials described inEmbodiment 2.

Examples of various modes of a storage battery are described below. Thedescription of the electrode 101 and the electrode 151 in FIGS. 13A and13B and the like can be referred to for storage batteries describedbelow. For example, the description of the electrode 101 in FIGS. 13Aand 13B can be referred to for a positive electrode 503 illustrated inFIG. 17, and the description of the electrode 151 in FIGS. 13A and 13Bcan be referred to for a negative electrode 506 illustrated in FIG. 17.

As another example of a wound storage battery, a cylindrical storagebattery is illustrated in FIGS. 16A and 16B. As illustrated in FIG. 16A,a cylindrical storage battery 600 includes a positive electrode cap(battery cap) 601 on the top surface and a battery can (outer can) 602on the side surface and bottom surface. The positive electrode cap 601and the battery can 602 are insulated from each other by a gasket(insulating gasket) 610.

FIG. 16B is a diagram schematically illustrating a cross section of thecylindrical storage battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with astripe-like separator 605 interposed therebetween is provided. Althoughnot illustrated, the battery element is wound around a center pin. Oneend of the battery can 602 is close and the other end thereof is open.For the battery can 602, a metal having a corrosion-resistant propertyto an electrolyte solution, such as nickel, aluminum, or titanium, analloy of such a metal, or an alloy of such a metal and another metal(e.g., stainless steel or the like) can be used. Alternatively, thebattery can 602 is preferably covered with nickel, aluminum, or the likein order to prevent corrosion due to the electrolyte solution. Insidethe battery can 602, the battery element in which the positiveelectrode, the negative electrode, and the separator are wound isinterposed between a pair of insulating plates 608 and 609 which faceeach other. Further, a nonaqueous electrolyte solution (not illustrated)is injected inside the battery can 602 provided with the batteryelement. As the nonaqueous electrolyte solution, a nonaqueouselectrolyte solution which is similar to that of the above coin-typestorage battery can be used.

Since the positive electrode and the negative electrode of thecylindrical storage battery are wound, active materials are preferablyformed on both sides of the current collectors. A positive electrodeterminal (positive electrode current collecting lead) 603 is connectedto the positive electrode 604, and a negative electrode terminal(negative electrode current collecting lead) 607 is connected to thenegative electrode 606. Both the positive electrode terminal 603 and thenegative electrode terminal 607 can be formed with a metal material suchas aluminum. The positive electrode terminal 603 and the negativeelectrode terminal 607 are resistance-welded to a safety valve mechanism612 and the bottom of the battery can 602, respectively. The safetyvalve mechanism 612 is electrically connected to the positive electrodecap 601 through a positive temperature coefficient (PTC) element 611.The safety valve mechanism 612 cuts off electrical connection betweenthe positive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery exceeds a predetermined thresholdvalue. Further, the PTC element 611, which serves as a thermallysensitive resistor whose resistance increases as temperature rises,limits the amount of current by increasing the resistance, in order toprevent abnormal heat generation. Note that barium titanate(BaTiO₃)-based semiconductor ceramic or the like can be used for the PTCelement.

In the case where an electrode is wound as in the cylindrical storagebattery illustrated in FIGS. 16A and 16B, a great stress is caused atthe time of winding the electrode. In addition, an outward stress froman axis of winding is applied to the electrode all the time in the casewhere a wound body of the electrode is stored in a housing. However, theactive material can be prevented from being cleaved even when such agreat stress is applied to the electrode.

<Thin Storage Battery>

FIG. 17 illustrates a thin storage battery as an example of a powerstorage device. When a flexible thin storage battery is used in anelectronic device at least part of which is flexible, the storagebattery can be bent as the electronic device is bent.

FIG. 17 is an external view of a thin storage battery 500. FIG. 18A is across-sectional view along dashed-dotted line A1-A2 in FIG. 17, and FIG.18B is a cross-sectional view along dashed-dotted line B1-B2 in FIG. 17.The thin storage battery 500 includes a positive electrode 503 includinga positive electrode current collector 501 and a positive electrodeactive material layer 502, a negative electrode 506 including a negativeelectrode current collector 504 and a negative electrode active materiallayer 505, a separator 507, an electrolyte solution 508, and an exteriorbody 509. The separator 507 is provided between the positive electrode503 and the negative electrode 506 in the exterior body 509. Theelectrolyte solution 508 is contained in the exterior body 509.

The separator 507 is preferably formed to have a bag-like shape tosurround one of the positive electrode 503 and the negative electrode506. For example, as illustrated in FIG. 19A, the separator 507 isfolded in two so that the positive electrode 503 is sandwiched, andsealed with a sealing member 514 in a region outside the regionoverlapping with the positive electrode 503; thus, the positiveelectrode 503 can be reliably supported inside the separator 507. Then,as illustrated in FIG. 19B, the positive electrodes 503 surrounded bythe separators 507 and the negative electrodes 506 are alternatelystacked and provided in the exterior body 509, whereby the thin storagebattery 500 can be formed.

Although the positive electrode lead electrode 510 and the negativeelectrode lead electrode 511 are provided on the same side in FIG. 17,the positive electrode lead electrode 510 and the negative electrodelead electrode 511 may be provided on different sides as illustrated inFIG. 20. The lead electrodes of a storage battery of one embodiment ofthe present invention can be freely positioned as described above;therefore, the degree of freedom in design is high. Accordingly, aproduct including a storage battery of one embodiment of the presentinvention can have a high degree of freedom in design. Furthermore, ayield of products each including a storage battery of one embodiment ofthe present invention can be increased.

The description of the exterior body 983 or the like can be referred tofor a material and a structure of the exterior body 509.

Although the examples in FIGS. 18A and 18B each include five positiveelectrode active material layer-negative electrode active material layerpairs (the positive and negative electrode active material layers ofeach pair face each other), it is needless to say that the number ofpairs of electrode active material layers is not limited to five, andmay be more than five or less than five. In the case of using a largenumber of electrode active material layers, the storage battery can havea high capacity. In contrast, in the case of using a small number ofelectrode active material layers, the storage battery can have a smallthickness and high flexibility.

Here, for example, the case where the positive electrode 503 illustratedin FIG. 17 includes the conductor 201 and the layer 102 which is formedover the conductor 201 and includes a positive electrode active materialis considered. In the storage battery 500 illustrated in each of FIGS.18A and 18B, the plurality of positive electrodes 503 are stacked. Here,the area of the conductor 201 included in the positive electrode 503 maybe, for example, greater than or equal to 1 mm² and less than or equalto 1 m², greater than or equal to 25 mm² and less than or equal to 1 m²,or greater than or equal to 100 mm² and less than or equal to 0.1 m².

Next, a variety of examples of the stack of the positive electrode, thenegative electrode, and the separator are described.

FIG. 21A illustrates an example where six positive electrodes 111 andsix negative electrodes 115 are stacked. One surface of a positiveelectrode current collector 121 included in a positive electrode 111 isprovided with a positive electrode active material layer 122. Onesurface of a negative electrode current collector 125 included in anegative electrode 115 is provided with a negative electrode activematerial layer 126.

In the structure illustrated in FIG. 21A, the positive electrodes 111and the negative electrodes 115 are stacked so that surfaces of thepositive electrodes 111 on each of which the positive electrode activematerial layer 122 is not provided are in contact with each other andthat surfaces of the negative electrodes 115 on each of which thenegative electrode active material layer 126 is not provided are incontact with each other. When the positive electrodes 111 and thenegative electrodes 115 are stacked in this manner, contact surfacesbetween metals can be formed; specifically, the surfaces of the positiveelectrodes 111 on each of which the positive electrode active materiallayer 122 is not provided can be in contact with each other, and thesurfaces of the negative electrodes 115 on each of which the negativeelectrode active material layer 126 is not provided can be in contactwith each other. The coefficient of friction of the contact surfacebetween metals can be lower than that of a contact surface between theactive material and the separator.

Therefore, when the secondary battery 10 is curved, the surfaces of thepositive electrodes 111 on each of which the positive electrode activematerial layer 122 is not provided slide on each other, and the surfacesof the negative electrodes 115 on each of which the negative electrodeactive material layer 126 is not provided slide on each other; thus, thestress due to the difference between the inner diameter and the outerdiameter of a curved portion can be relieved. Here, the inner diameterof the curved portion refers to the radius of curvature of the innersurface of the curved portion in the exterior body 509 of the thinstorage battery 500 in the case where the thin storage battery 500 iscurved, for example. Therefore, the deterioration of the thin storagebattery 500 can be inhibited. Furthermore, the thin storage battery 500can have high reliability.

FIG. 21B illustrates an example of a stacked-layer structure of thepositive electrodes 111 and the negative electrodes 115 which isdifferent from that in FIG. 21A. The structure illustrated in FIG. 21Bis different from that in FIG. 21A in that the positive electrode activematerial layers 122 are provided on both surfaces of the positiveelectrode current collector 121. When the positive electrode activematerial layers 122 are provided on both the surfaces of the positiveelectrode current collector 121 as illustrated in FIG. 21B, the capacityper unit volume of the thin storage battery 500 can be increased.

FIG. 21C illustrates an example of a stack of the positive electrodes111 and the negative electrodes 115 which is different from that in FIG.21B. The structure illustrated in FIG. 21C is different from that inFIG. 21B in that the negative electrode active material layers 126 areprovided on both surfaces of the negative electrode current collector125. When the negative electrode active material layers 126 are providedon both the surfaces of the negative electrode current collector 125 asillustrated in FIG. 21C, the capacity per unit volume of the thinstorage battery 500 can be further increased.

In the structures illustrated in FIGS. 21A to 21C, the separator 123 hasa bag-like shape by which the positive electrodes 111 are surrounded;however, one embodiment of the present invention is not limited thereto.FIG. 22A illustrates an example in which the separator 123 has adifferent structure from that in FIG. 21A. The structure illustrated inFIG. 22A is different from that in FIG. 21A in that the separator 123,which is sheet-like, is provided between every pair of the positiveelectrode active material layer 122 and the negative electrode activematerial layer 126. In the structure illustrated in FIG. 22A, sixpositive electrodes 111 and six negative electrodes 115 are stacked, andsix separators 123 are provided.

FIG. 22B illustrates an example in which the separator 123 differentfrom that in FIG. 22A is provided. The structure illustrated in FIG. 22Bis different from that in FIG. 22A in that one sheet of separator 123 isfolded more than once to be interposed between every pair of thepositive electrode active material layer 122 and the negative electrodeactive material layer 126. It can be said that the structure illustratedin FIG. 22B is a structure in which the separators 123 in the respectivelayers which are illustrated in FIG. 22A are extended and connectedtogether between the layers. In the structure of FIG. 22B, six positiveelectrodes 111 and six negative electrodes 115 are stacked and thus theseparator 123 is folded at least five times. The separator 123 is notnecessarily provided so as to be interposed between every pair of thepositive electrode active material layer 122 and the negative electrodeactive material layer 126, and the plurality of positive electrodes 111and the plurality of negative electrodes 115 may be bound together byextending the separator 123.

Note that the positive electrode, the negative electrode, and theseparator may be stacked as illustrated in FIGS. 23A to 23C. FIG. 23A isacross-sectional view of a first electrode assembly 130, and FIG. 23B isa cross-sectional view of a second electrode assembly 131. FIG. 23C is across-sectional view taken along the dashed-dotted line A1-A2 in FIG.1A. In FIG. 23C, the first electrode assembly 130, the second electrodeassembly 131, and the separator 123 are selectively illustrated for thesake of clarity.

As illustrated in FIG. 23C, the thin storage battery 500 includes aplurality of first electrode assemblies 130 and a plurality of secondelectrode assemblies 131.

As illustrated in FIG. 23A, in each of the first electrode assemblies130, a positive electrode 111 a including the positive electrode activematerial layers 122 on both surfaces of a positive electrode currentcollector 121, the separator 123, a negative electrode 115 a includingthe negative electrode active material layers 126 on both surfaces of anegative electrode current collector 125, the separator 123, and thepositive electrode 111 a including the positive electrode activematerial layers 122 on both surfaces of the positive electrode currentcollector 121 are stacked in this order. As illustrated in FIG. 23B, ineach of the second electrode assemblies 131, the negative electrode 115a including the negative electrode active material layers 126 on bothsurfaces of the negative electrode current collector 125, the separator123, the positive electrode 111 a including the positive electrodeactive material layers 122 on both surfaces of the positive electrodecurrent collector 121, the separator 123, and the negative electrode 115a including the negative electrode active material layers 126 on bothsurfaces of the negative electrode current collector 125 are stacked inthis order.

As illustrated in FIG. 23C, the plurality of first electrode assemblies130 and the plurality of second electrode assemblies 131 are coveredwith the wound separator 123.

<Coin-Type Storage Battery>

An example of a coin-type storage battery is described with reference toFIGS. 24A and 24B. FIG. 24A is an external view of a coin-type(single-layer flat type) storage battery, and FIG. 24B isacross-sectional view thereof. As an exterior body of the coin-typestorage battery, a positive electrode can and a negative electrode canare used.

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305.

A negative electrode 307 includes a negative electrode current collector308 and a negative electrode active material layer 309 provided incontact with the negative electrode current collector 308.

Note that only one surface of each of the positive electrode 304 and thenegative electrode 307 used for the coin-type storage battery 300 isprovided with an active material layer.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolyte solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel or thelike) can be used. Alternatively, it is preferable to cover the positiveelectrode can 301 and the negative electrode can 302 with nickel,aluminum, or the like in order to prevent corrosion due to theelectrolyte solution. The positive electrode can 301 and the negativeelectrode can 302 are electrically connected to the positive electrode304 and the negative electrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte solution. Then, asillustrated in FIG. 24B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303interposed therebetween. In such a manner, the coin-type storage battery300 can be manufactured.

<Curvature of Exterior Body>

The exterior body of the power storage device can change its form suchthat the smallest curvature radius is greater than or equal to 3 mm andless than or equal to 30 mm, preferably greater than or equal to 3 mmand less than or equal to 10 mm. One or two films are used as theexterior body of the power storage device. The power storage device hasa cross section sandwiched by two curved surfaces of the films when itis bent.

Description will be given of the radius of curvature of a surface withreference to FIGS. 25A to 25C. In FIG. 25A, on a plane 1701 along whicha curved surface 1700 is cut, part of a curve 1702 of the curved surface1700 is approximate to an arc of a circle, and the radius of the circleis referred to as a radius 1703 of curvature and the center of thecircle is referred to as a center 1704 of curvature. FIG. 25B is a topview of the curved surface 1700. FIG. 25C is a cross-sectional view ofthe curved surface 1700 taken along the plane 1701. When a curvedsurface is cut by a plane, the radius of curvature of a curve in a crosssection differs depending on the angle between the curved surface andthe plane or on the cut position, and the smallest radius of curvatureis defined as the radius of curvature of a surface in this specificationand the like.

In the case of bending a power storage device in which a component 1805including electrodes and an electrolyte solution is sandwiched betweentwo films as exterior bodies, a radius 1802 of curvature of a film 1801close to a center 1800 of curvature of the power storage device issmaller than a radius 1804 of curvature of a film 1803 far from thecenter 1800 of curvature (FIG. 26A). When the power storage device iscurved and has an arc-shaped cross section, compressive stress isapplied to a surface of the film on the side closer to the center 1800of curvature and tensile stress is applied to a surface of the film onthe side farther from the center 1800 of curvature (FIG. 26B). However,by forming a pattern including projections or depressions on surfaces ofthe exterior bodies, the influence of a strain can be reduced to beacceptable even when compressive stress and tensile stress are applied.For this reason, the power storage device can change its form such thatthe exterior body on the side closer to the center of curvature has thesmallest curvature radius greater than or equal to 3 mm and less than orequal to 30 mm, preferably greater than or equal to 3 mm and less thanor equal to 10 mm.

Note that the cross-sectional shape of the power storage device is notlimited to a simple arc shape, and the cross section can be partlyarc-shaped; for example, a shape illustrated in FIG. 26C, a wavy shapeillustrated in FIG. 26D, or an S shape can be used. In the case wherethe curvature radius is changed by application of external force to thepower storage device, the range of the curvature radius may be greaterthan or equal to 3 mm and less than or equal to 30 mm, for example. Forexample, the curvature radius of the power storage device which is bentto maximum, that is the smallest curvature radius, is preferably greaterthan or equal to 3 mm and less than or equal to 10 mm, and furtherpreferably greater than or equal to 3 mm and less than or equal to 6 mm.Here, in the case where the curved surfaces of the power storage devicehave a plurality of curvature centers, the curvature radius of the powerstorage device may be the smallest curvature radius of one of the twoexternal bodies whose curvature center is closer to the curvature centerin the curved surface having the smallest curvature radius amongcurvature radii of the plurality of curvature centers.

<Curvature of Conductor>

The conductor of one embodiment of the present invention has asheet-like shape and can be changed in its shape so that the smallestcurvature radius is, for example, greater than or equal to 3 mm and lessthan or equal to 30 mm, and further preferably greater than or equal to3 mm and less than or equal to 10 mm. Accordingly, the conductor of oneembodiment of the present invention is less likely to be cracked bybeing wound or bent, for example. Thus, the conductor of one embodimentof the present invention is suitable for a wound power storage device.Furthermore, the conductor of one embodiment of the present inventioncan be changed in its shape in accordance with a change in shape of thepower storage device by external force. Accordingly, when the conductorof one embodiment of the present invention is used for a flexible powerstorage device, the power storage device can have higher reliability.

<Structural Example of Power Storage System>

Structural examples of power storage systems will be described withreference to FIGS. 27A and 27B to FIGS. 29A and 29B. Here, a powerstorage system refers to, for example, a device including a powerstorage device.

FIGS. 27A and 27B are external views of a power storage system. Thepower storage system includes a circuit board 900 and a storage battery913. A label 910 is attached to the storage battery 913. As shown inFIG. 27B, the power storage system further includes a terminal 951, aterminal 952, an antenna 914, and an antenna 915.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. The shape of each of the antennas 914 and 915 is not limited to acoil shape and may be a linear shape or a plate shape. Further, a planarantenna, an aperture antenna, a traveling-wave antenna, an EH antenna, amagnetic-field antenna, or a dielectric antenna may be used.Alternatively, the antenna 914 or the antenna 915 may be a flat-plateconductor. The flat-plate conductor can serve as one of conductors forelectric field coupling. That is, the antenna 914 or the antenna 915 canserve as one of two conductors of a capacitor. Thus, electric power canbe transmitted and received not only by an electromagnetic field or amagnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The power storage system includes a layer 916 between the storagebattery 913 and the antennas 914 and 915. The layer 916 may have afunction of blocking an electromagnetic field by the storage battery913. As the layer 916, for example, a magnetic body can be used.

Note that the structure of the power storage system is not limited tothat shown in FIGS. 27A and 27B.

For example, as shown in FIGS. 28A1 and 28A2, two opposite surfaces ofthe storage battery 913 in FIGS. 27A and 27B may be provided withrespective antennas.

FIG. 28A1 is an external view showing one side of the opposite surfaces,and FIG. 28A2 is an external view showing the other side of the oppositesurfaces. For portions similar to those in FIGS. 27A and 27B, thedescription of the power storage system illustrated in FIGS. 27A and 27Bcan be referred to as appropriate.

As illustrated in FIG. 28A1, the antenna 914 is provided on one of theopposite surfaces of the storage battery 913 with the layer 916interposed therebetween, and as illustrated in FIG. 28A2, the antenna915 is provided on the other of the opposite surfaces of the storagebattery 913 with a layer 917 interposed therebetween. The layer 917 mayhave a function of preventing an adverse effect on an electromagneticfield by the storage battery 913. As the layer 917, for example, amagnetic body can be used.

With the above structure, both of the antennas 914 and 915 can beincreased in size.

Alternatively, as illustrated in FIGS. 28B1 and 28B2, two oppositesurfaces of the storage battery 913 in FIGS. 27A and 27B may be providedwith different types of antennas. FIG. 28B1 is an external view showingone side of the opposite surfaces, and FIG. 28B2 is an external viewshowing the other side of the opposite surfaces. For portions similar tothose in FIGS. 27A and 27B, the description of the power storage systemillustrated in FIGS. 27A and 27B can be referred to as appropriate.

As illustrated in FIG. 28B1, the antennas 914 and 915 are provided onone of the opposite surfaces of the storage battery 913 with the layer916 provided between the storage battery 913 and the antennas 914 and915, and as illustrated in FIG. 28B2, an antenna 918 is provided on theother of the opposite surfaces of the storage battery 913 with the layer917 provided therebetween. The antenna 918 has a function ofcommunicating data with an external device, for example. An antenna witha shape that can be applied to the antennas 914 and 915, for example,can be used as the antenna 918. As a system for communication using theantenna 918 between the power storage system and another device, aresponse method that can be used between the power storage system andanother device, such as NFC, can be employed.

Alternatively, as illustrated in FIG. 29A, the storage battery 913 inFIGS. 27A and 27B may be provided with a display device 920. The displaydevice 920 is electrically connected to the terminal 911 via a terminal919. It is possible that the label 910 is not provided in a portionwhere the display device 920 is provided. For portions similar to thosein FIGS. 27A and 27B, the description of the power storage systemillustrated in FIGS. 27A and 27B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether charge is being carried out, an image showing the amount ofstored power, or the like. As the display device 920, electronic paper,a liquid crystal display device, an electroluminescent (EL) displaydevice, or the like can be used. For example, the use of electronicpaper can reduce power consumption of the display device 920.

Alternatively, as illustrated in FIG. 29B, the storage battery 913illustrated in FIGS. 27A and 27B may be provided with a sensor 921. Thesensor 921 is electrically connected to the terminal 911 via a terminal922. For portions similar to those in FIGS. 27A and 27B, the descriptionof the power storage system illustrated in FIGS. 27A and 27B can bereferred to as appropriate.

As the sensor 921, a sensor that has a function of measuring, forexample, force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,electric current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared rays can be used.With the sensor 921, for example, data on an environment (e.g.,temperature) where the power storage system is placed can be determinedand stored in a memory inside the circuit 912.

The electrode of one embodiment of the present invention is used in thestorage battery and the power storage system that are described in thisembodiment. The capacity of the storage battery and the power storagesystem can thus be high. Furthermore, energy density can be high.Moreover, reliability can be high, and life can be long.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 4

In this embodiment, an example of an electronic device including aflexible power storage device will be described.

FIGS. 30A to 30G illustrate examples of electronic devices including theflexible power storage devices described in Embodiment 2. Examples ofelectronic devices each including a flexible power storage deviceinclude television devices (also referred to as televisions ortelevision receivers), monitors of computers or the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as mobile phones or mobile phonedevices), portable game machines, portable information terminals, audioreproducing devices, and large game machines such as pachinko machines.

In addition, a flexible power storage device can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of a car.

FIG. 30A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage device 7407.

FIG. 30B illustrates the mobile phone 7400 that is bent. When the wholemobile phone 7400 is bent by the external force, the power storagedevice 7407 included in the mobile phone 7400 is also bent. FIG. 30Cillustrates the bent power storage device 7407. The power storage device7407 is a thin storage battery. The power storage device 7407 is fixedin a state of being bent. Note that the power storage device 7407includes a lead electrode 7408 electrically connected to a currentcollector 7409. The current collector 7409 is, for example, copper foil,and partly alloyed with gallium; thus, adhesion between the currentcollector 7409 and an active material layer in contact with the currentcollector 7409 is improved and the power storage device 7407 can havehigh reliability even in a state of being bent.

FIG. 30D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a power storage device 7104. FIG. 30Eillustrates the bent power storage device 7104. When the display deviceis worn on a user's arm while the power storage device 7104 is bent, thehousing changes its form and the curvature of a part or the whole of thepower storage device 7104 is changed. Note that the radius of curvatureof a curve at a point refers to the radius of the circular arc that bestapproximates the curve at that point. The reciprocal of the radius ofcurvature is curvature. Specifically, a part or the whole of the housingor the main surface of the power storage device 7104 is changed in therange of radius of curvature from 40 mm to 150 mm inclusive. When theradius of curvature at the main surface of the power storage device 7104is greater than or equal to 40 mm and less than or equal to 150 mm, thereliability can be kept high.

FIG. 30F illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting texts, music reproduction, Internet communication, and acomputer game.

The display surface of the display portion 7202 is curved, and imagescan be displayed on the curved display surface. In addition, the displayportion 7202 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,by touching an icon 7207 displayed on the display portion 7202,application can be started.

With the operation button 7205, a variety of functions such as timesetting, power on/off, on/off of wireless communication, setting andcancellation of a silent mode, and setting and cancellation of a powersaving mode can be performed. For example, the functions of theoperation button 7205 can be set freely by setting the operation systemincorporated in the portable information terminal 7200.

The portable information terminal 7200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 7200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. In addition,charging via the input output terminal 7206 is possible. Note that thecharging operation may be performed by wireless power feeding withoutusing the input output terminal 7206.

The display portion 7202 of the portable information terminal 7200 isprovided with a power storage device including the electrode member ofone embodiment of the present invention. For example, the power storagedevice 7104 illustrated in FIG. 30E that is in the state of being curvedcan be provided in the housing 7201. Alternatively, the power storagedevice 7104 illustrated in FIG. 30E can be provided in the band 7203such that it can be curved.

A portable information terminal 7200 preferably includes a sensor. Asthe sensor, for example a human body sensor such as a fingerprintsensor, a pulse sensor, or a temperature sensor, a touch sensor, apressure sensitive sensor, an acceleration sensor, or the like ispreferably mounted.

FIG. 30G illustrates an example of an armband display device. A displaydevice 7300 includes a display portion 7304 and the power storage deviceof one embodiment of the present invention. The display device 7300 caninclude a touch sensor in the display portion 7304 and can serve as aportable information terminal.

The display surface of the display portion 7304 is bent, and images canbe displayed on the bent display surface. A display state of the displaydevice 7300 can be changed by, for example, near field communication,which is a communication method based on an existing communicationstandard.

The display device 7300 includes an input output terminal, and data canbe directly transmitted to and received from another informationterminal via a connector. In addition, charging via the input outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input output terminal.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 5

In this embodiment, examples of electronic devices that can includepower storage devices will be described.

FIGS. 31A and 31B illustrate an example of a tablet terminal that can befolded in half A tablet terminal 9600 illustrated in FIGS. 31A and 31Bincludes a housing 9630 a, a housing 9630 b, a movable portion 9640connecting the housings 9630 a and 9630 b, a display portion 9631including a display portion 9631 a and a display portion 9631 b, adisplay mode changing switch 9626, a power switch 9627, a power savingmode changing switch 9625, a fastener 9629, and an operation switch9628. FIG. 31A illustrates the tablet terminal 9600 that is opened, andFIG. 31B illustrates the tablet terminal 9600 that is closed.

The tablet terminal 9600 includes a power storage unit 9635 inside thehousings 9630 a and 9630 b. The power storage unit 9635 is providedacross the housings 9630 a and 9630 b, passing through the movableportion 9640.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 31A shows, as an example, that half of thearea of the display portion 9631 a has only a display function and theother half of the area has a touch panel function. However, thestructure of the display portion 9631 a is not limited to this, and allthe area of the display portion 9631 a may have a touch panel function.For example, all the area of the display portion 9631 a can display akeyboard and serve as a touch panel while the display portion 9631 b canbe used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode changing switch 9626 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power saving mode changing switch 9625 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal 9600, which is measured with anoptical sensor incorporated in the tablet terminal 9600. In addition tothe optical sensor, other detecting devices such as sensors fordetermining inclination, such as a gyroscope or an acceleration sensor,may be incorporated in the tablet terminal.

Although the display portion 9631 a and the display portion 9631 b havethe same area in FIG. 31A, one embodiment of the present invention isnot limited to this example. The display portion 9631 a and the displayportion 9631 b may have different areas or different display quality.For example, one of the display portions 9631 a and 9631 b may displayhigher definition images than the other.

The tablet terminal is closed in FIG. 31B. The tablet terminal includesthe housing 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634 including a DCDC converter 9636. The power storage unit ofone embodiment of the present invention is used as the power storageunit 9635.

The tablet terminal 9600 can be folded such that the housings 9630 a and9630 b overlap with each other when not in use. Thus, the displayportions 9631 a and 9631 b can be protected, which increases thedurability of the tablet terminal 9600. In addition, the power storageunit 9635 of one embodiment of the present invention has flexibility andcan be repeatedly bent without a significant decrease in charge anddischarge capacity. Thus, a highly reliable tablet terminal can beprovided.

The tablet terminal illustrated in FIGS. 31A and 31B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, or the time on the display portion, a touch-input function ofoperating or editing data displayed on the display portion by touchinput, a function of controlling processing by various kinds of software(programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processing portion, and the like. Note that the solarcell 9633 can be provided on one or both surfaces of the housing 9630and the power storage unit 9635 can be charged efficiently. The use of alithium-ion battery as the power storage unit 9635 brings an advantagesuch as reduction in size.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 31B will be described with reference to a blockdiagram in FIG. 31C. The solar cell 9633, the power storage unit 9635,the DCDC converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 31C, and the power storageunit 9635, the DCDC converter 9636, the converter 9637, and the switchesSW1 to SW3 correspond to the charge and discharge control circuit 9634in FIG. 31B.

First, an example of operation when electric power is generated by thesolar cell 9633 using external light will be described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDCDC converter 9636 to a voltage for charging the power storage unit9635. When the display portion 9631 is operated with the electric powerfrom the solar cell 9633, the switch SW Iis turned on and the voltage ofthe electric power is raised or lowered by the converter 9637 to avoltage needed for operating the display portion 9631. When display onthe display portion 9631 is not performed, the switch SW1 is turned offand the switch SW2 is turned on, so that the power storage unit 9635 canbe charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, one embodiment of the present invention isnot limited to this example. The power storage unit 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module capable of performing charging by transmitting andreceiving electric power wirelessly (without contact), or any of theother charge means used in combination.

FIG. 32 illustrates other examples of electronic devices. In FIG. 32, adisplay device 8000 is an example of an electronic device including apower storage device 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, and the power storage device 8004.The power storage device 8004 of one embodiment of the present inventionis provided in the housing 8001. The display device 8000 can receiveelectric power from a commercial power supply. Alternatively, thedisplay device 8000 can use electric power stored in the power storagedevice 8004. Thus, the display device 8000 can be operated with the useof the power storage device 8004 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 32, an installation lighting device 8100 is an example of anelectronic device including a power storage device 8103 of oneembodiment of the present invention. Specifically, the lighting device8100 includes a housing 8101, a light source 8102, and the power storagedevice 8103. Although FIG. 32 illustrates the case where the powerstorage device 8103 is provided in a ceiling 8104 on which the housing8101 and the light source 8102 are installed, the power storage device8103 may be provided in the housing 8101. The lighting device 8100 canreceive electric power from a commercial power supply. Alternatively,the lighting device 8100 can use electric power stored in the powerstorage device 8103. Thus, the lighting device 8100 can be operated withthe use of power storage device 8103 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 32 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the power storage device of one embodiment of the presentinvention can be used in a tabletop lighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, and alight-emitting element such as an LED or an organic EL element are givenas examples of the artificial light source.

In FIG. 32, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including apower storage device 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, and the power storage device 8203. Although FIG. 32illustrates the case where the power storage device 8203 is provided inthe indoor unit 8200, the power storage device 8203 may be provided inthe outdoor unit 8204. Alternatively, the power storage devices 8203 maybe provided in both the indoor unit 8200 and the outdoor unit 8204. Theair conditioner can receive electric power from a commercial powersupply. Alternatively, the air conditioner can use electric power storedin the power storage device 8203. Particularly in the case where thepower storage devices 8203 are provided in both the indoor unit 8200 andthe outdoor unit 8204, the air conditioner can be operated with the useof the power storage device 8203 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 32 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 32, an electric refrigerator-freezer 8300 is an example of anelectronic device including a power storage device 8304 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a door for arefrigerator 8302, a door for a freezer 8303, and the power storagedevice 8304. The power storage device 8304 is provided in the housing8301 in FIG. 32. The electric refrigerator-freezer 8300 can receiveelectric power from a commercial power supply. Alternatively, theelectric refrigerator-freezer 8300 can use electric power stored in thepower storage device 8304. Thus, the electric refrigerator-freezer 8300can be operated with the use of the power storage device 8304 of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

Note that among the electronic devices described above, a high-frequencyheating apparatus such as a microwave oven and an electronic device suchas an electric rice cooker require high power in a short time. Thetripping of a breaker of a commercial power supply in use of anelectronic device can be prevented by using the power storage device ofone embodiment of the present invention as an auxiliary power supply forsupplying electric power which cannot be supplied enough by a commercialpower supply.

In addition, in a time period when electronic devices are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power supply source (such a proportionreferred to as a usage rate of electric power) is low, electric powercan be stored in the power storage device, whereby the usage rate ofelectric power can be reduced in a time period when the electronicdevices are used. For example, in the case of the electricrefrigerator-freezer 8300, electric power can be stored in the powerstorage device 8304 in night time when the temperature is low and thedoor for a refrigerator 8302 and the door for a freezer 8303 are notoften opened or closed. On the other hand, in daytime when thetemperature is high and the door for a refrigerator 8302 and the doorfor a freezer 8303 are frequently opened and closed, the power storagedevice 8304 is used as an auxiliary power supply; thus, the usage rateof electric power in daytime can be reduced.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 6

In this embodiment, examples of vehicles using power storage deviceswill be described.

The use of power storage devices in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 33A and 33B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8400 illustrated inFIG. 33A is an electric vehicle that runs on the power of an electricmotor. Alternatively, the automobile 8400 is a hybrid electric vehiclecapable of driving appropriately using either an electric motor or anengine. One embodiment of the present invention can provide ahigh-mileage vehicle. The automobile 8400 includes the power storagedevice. The power storage device is used not only for driving theelectric motor 8406, but also for supplying electric power to alight-emitting device such as a headlight 8401 or a room light (notillustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8400. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8400, such as a navigation system.

FIG. 33B illustrates an automobile 8500 including the power storagedevice. The automobile 8500 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.33B, a power storage device 8024 included in the automobile 8500 ischarged with the use of a ground-based charging apparatus 8021 through acable 8022. In charging, a given method such as CHAdeMO (registeredtrademark) or Combined Charging System may be employed as a chargingmethod, the standard of a connector, or the like as appropriate. Theground-based charging apparatus 8021 may be a charging station providedin a commerce facility or a power source in a house. For example, withthe use of a plug-in technique, the power storage device 8024 includedin the automobile 8500 can be charged by being supplied with electricpower from outside. The charging can be performed by converting ACelectric power into DC electric power through a converter such as anAC-DC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. Furthermore, a solar cell may be provided in theexterior of the automobile to charge the power storage device when theautomobile stops or moves. To supply electric power in such acontactless manner, an electromagnetic induction method or a magneticresonance method can be used.

According to one embodiment of the present invention, the power storagedevice can have improved cycle characteristics and reliability.Furthermore, according to one embodiment of the present invention, thepower storage device itself can be made more compact and lightweight asa result of improved characteristics of the power storage device. Thecompact and lightweight power storage device contributes to a reductionin the weight of a vehicle, and thus increases the driving distance.Furthermore, the power storage device included in the vehicle can beused as a power source for supplying electric power to products otherthan the vehicle. In such a case, the use of a commercial power sourcecan be avoided at peak time of electric power demand.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 6

A battery management unit (BMU) that can be used in combination withbattery cells each including the materials described in the aboveembodiment and transistors that are suitable for a circuit included inthe battery management unit will be described with reference to FIG. 34to FIG. 40. In this embodiment, in particular, a battery management unitof a power storage device including battery cells connected in serieswill be described.

When a plurality of battery cells connected in series is charged anddischarged repeatedly, each battery cell has different capacity (outputvoltage) from one another due to the variation in characteristics amongthe battery cells. A discharge capacity of all of the plurality ofbattery cells connected in series depends on a battery cell with smallcapacity. The variation in capacities among the battery cells reducesthe capacity of the all the battery cells at the time of discharging.Charging based on a battery cell with small capacity may causeinsufficient charging. Charging based on a battery cell with highcapacity may cause overcharge.

Thus, the battery management unit of the power storage device includingthe battery cells connected in series has a function of reducingvariations in capacity among the battery cells, which cause anundercharge and an overcharge. Examples of a circuit configuration forreducing variations in capacity among battery cells include a resistivetype, a capacitive type, and an inductive type, and a circuitconfiguration that can reduce variations in capacity among battery cellsusing transistors with a low off-state current will be explained here asan example.

A transistor including an oxide semiconductor in its channel formationregion (an OS transistor) is preferably used as the transistor with alow off-state current. When an OS transistor with a low off-statecurrent is used in the circuit of the battery management unit of thepower storage device, the amount of charge that leaks from a battery canbe reduced, and reduction in capacity with the lapse of time can besuppressed.

As the oxide semiconductor used in the channel formation region, anIn-M-Zn oxide (M is Ga, Sn, Y, Zr, La, Ce, or Nd) is used. In the casewhere the atomic ratio of the metal elements of a target for forming anoxide semiconductor film is In M:Zn=x₁:y₁:z₁, x₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, more preferablygreater than or equal to 1 and less than or equal to 6, and z₁/y₁ ispreferably greater than or equal to ⅓ and less than or equal to 6, morepreferably greater than or equal to 1 and less than or equal to 6. Notethat when z₁/y₁ is greater than or equal to 1 and less than or equal to6, a CAAC-OS film as the oxide semiconductor film is easily formed.

Here, the details of the CAAC-OS film will be described.

A CAAC-OS film is one of oxide semiconductor films having a plurality ofc-axis aligned crystal parts.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OSfilm, which is obtained using a transmission electron microscope (TEM),a plurality of crystal parts can be observed. However, in thehigh-resolution TEM image, a boundary between crystal parts, that is, agrain boundary is not clearly observed. Thus, in the CAAC-OS film, areduction in electron mobility due to the grain boundary is less likelyto occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in the direction substantially parallel to thesample surface, metal atoms are arranged in a layered manner in thecrystal parts. Each metal atom layer reflects unevenness of a surfaceover which the CAAC-OS film is formed (hereinafter, a surface over whichthe CAAC-OS film is formed is referred to as a formation surface) or thetop surface of the CAAC-OS film, and is arranged parallel to theformation surface or the top surface of the CAAC-OS film.

On the other hand, according to the plan high-resolution TEM image ofthe CAAC-OS film observed in the direction substantially perpendicularto the sample surface, metal atoms are arranged in a triangular orhexagonal arrangement in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

For example, when the structure of a CAAC-OS including an InGaZnO₄crystal is analyzed by an out-of-plane method using an X-ray diffraction(XRD) apparatus, a peak may appear at a diffraction angle (2θ) of around31°. This peak is derived from the (009) plane of the InGaZnO₄ crystal,which indicates that crystals in the CAAC-OS film have c-axis alignment,and that the c-axes are aligned in the direction substantiallyperpendicular to the formation surface or the top surface of the CAAC-OSfilm.

Note that in analysis of the CAAC-OS film by an out-of-plane method,another peak may appear when 20 is around 36°, in addition to the peakat 2θ of around 31°. The peak at 2θ of around 36 indicates that acrystal having no c-axis alignment is included in part of the CAAC-OSfilm. It is preferable that in the CAAC-OS film, a peak appear when 2θis around 310 and that a peak not appear when 2θ is around 36°.

The CAAC-OS film is an oxide semiconductor film with low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element (specifically,silicon or the like) having higher strength of bonding to oxygen than ametal element included in an oxide semiconductor film extracts oxygenfrom the oxide semiconductor film, which results in disorder of theatomic arrangement and reduced crystallinity of the oxide semiconductorfilm. Furthermore, a heavy metal such as iron or nickel, argon, carbondioxide, or the like has a large atomic radius (molecular radius), andthus disturbs the atomic arrangement of the oxide semiconductor film andcauses a decrease in crystallinity when it is contained in the oxidesemiconductor film. Note that the impurity contained in the oxidesemiconductor might serve as a carrier trap or a carrier generationsource.

The CAAC-OS film is an oxide semiconductor having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein, for example.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially intrinsic” state. Ahighly purified intrinsic or substantially intrinsic oxide semiconductorhas few carrier generation sources, and thus can have a low carrierdensity. Therefore, a transistor including the oxide semiconductor filmrarely has negative threshold voltage (is rarely normally on). Thehighly purified intrinsic or substantially intrinsic oxide semiconductorfilm has few carrier traps. Accordingly, the transistor including theoxide semiconductor film has little variation in electricalcharacteristics and high reliability. Charge trapped by the carriertraps in the oxide semiconductor film takes a long time to be releasedand might behave like fixed charge. Thus, the transistor including theoxide semiconductor film having high impurity concentration and a highdensity of defect states has unstable electrical characteristics in somecases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Since the OS transistor has a wider band gap than a transistor includingsilicon in its channel formation region (a Si transistor), dielectricbreakdown is unlikely to occur when a high voltage is applied. Althougha voltage of several hundreds of volts is generated when battery cellsare connected in series, the above-described OS transistor is suitablefor a circuit of a battery management unit which is used for suchbattery cells in the power storage device.

FIG. 34 is an example of a block diagram of the power storage device. Apower storage device BT00 illustrated in FIG. 34 includes a terminalpair BT01, a terminal pair BT02, a switching control circuit BT03, aswitching circuit BT04, a switching circuit BT05, a voltagetransformation control circuit BT06, a voltage transformer circuit BT07,and a battery portion BT08 including a plurality of battery cells BT09connected in series.

In the power storage device BT00 illustrated in FIG. 34, a portionincluding the terminal pair BT01, the terminal pair BT02, the switchingcontrol circuit BT03, the switching circuit BT04, the switching circuitBT05, the voltage transformation control circuit BT06, and the voltagetransformer circuit BT07 can be referred to as a battery managementunit.

The switching control circuit BT03 controls operations of the switchingcircuits BT04 and BT05. Specifically, the switching control circuit BT03selects battery cells to be discharged (a discharge battery cell group)and battery cells to be charged (a charge battery cell group) inaccordance with voltage measured for every battery cell BT09.

Furthermore, the switching control circuit BT03 outputs a control signalS1 and a control signal S2 on the basis of the selected dischargebattery cell group and the selected charge battery cell group. Thecontrol signal S1 is output to the switching circuit BT04. The controlsignal S1 controls the switching circuit BT04 so that the terminal pairBT01 and the discharge battery cell group are connected. In addition,the control signal S2 is output to the switching circuit BT05. Thecontrol signal S2 controls the switching circuit BT05 so that theterminal pair BT02 and the charge battery cell group are connected.

The switching control circuit BT03 generates the control signal Si andthe control signal S2 on the basis of the connection relation of theswitching circuit BT4, the switching circuit BT05, and the voltagetransformer circuit BT07 so that terminals having the same polarity ofthe terminal pair BT01 and the discharge battery cell group areconnected with each other, or terminals having the same polarity of theterminal pair BT02 and the charge battery cell group are connected witheach other.

The operations of the switching control circuit BT03 will be describedin detail.

First, the switching control circuit BT03 measures the voltage of eachof the plurality of battery cells BT09. Then, the switching controlcircuit BT03 determines that the battery cell BT09 having a voltagehigher than a predetermined threshold value is a high-voltage batterycell (high-voltage cell) and that the battery cell BT09 having a voltagelower than the predetermined threshold value is a low-voltage batterycell (low-voltage cell), for example.

As a method to determine whether a battery cell is a high-voltage cellor a low-voltage cell, any of various methods can be employed. Forexample, the switching control circuit BT03 may determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell on thebasis of the voltage of the battery cell BT09 having the highest voltageor the lowest voltage among the plurality of battery cells BT9. In thiscase, the switching control circuit BT03 can determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell by, forexample, determining whether or not the ratio of the voltage of eachbattery cell BT09 to the reference voltage is the predetermined value ormore. Then, the switching control circuit BT03 determines a chargebattery cell group and a discharge battery cell group on the basis ofthe determination result.

Note that high-voltage cells and low-voltage cells are mixed in variousstates in the plurality of battery cells BT09. For example, theswitching control circuit BT03 selects a portion having the largestnumber of high-voltage cells connected in series as the dischargebattery cell group of mixed high-voltage cells and low-voltage cells.Furthermore, the switching control circuit BT03 selects a portion havingthe largest number of low-voltage cells connected in series as thecharge battery cell group. In addition, the switching control circuitBT03 may preferentially select the battery cells BT09 which are almostovercharged or over-discharged as the discharge battery cell group orthe charge battery cell group.

Here, operation examples of the switching control circuit BT03 in thisembodiment will be described with reference to FIGS. 35A to 35C. FIGS.35A to 35C illustrate the operation examples of the switching controlcircuit BT03. Note that FIGS. 35A to 35C each illustrate the case wherefour battery cells BT09 are connected in series as an example forconvenience of explanation.

FIG. 35A shows the case where the relation of voltages Va, Vb, Vc, andVd is Va=Vb=Vc>Vd where the voltages Va, Vb, Vc, and Vd are the voltagesof a battery cell a, a battery cell b, a battery cell c, and a batterycell d, respectively. That is, a series of three high-voltage cells a toc and one low-voltage cell d are connected in series. In this case, theswitching control circuit BT03 selects the series of three high-voltagecells a to c as the discharge battery cell group. In addition, theswitching control circuit BT03 selects the low-voltage cell d as thecharge battery cell group.

Next, FIG. 35B shows the case where the relation of the voltages isVc>Va=Vb>>Vd. That is, a series of two low-voltage cells a and b, onehigh-voltage cell c, and one low-voltage cell d which is almostover-discharged are connected in series. In this case, the switchingcontrol circuit BT03 selects the high-voltage cell c as the dischargebattery cell group. Since the low-voltage cell d is almostover-discharged, the switching control circuit BT03 preferentiallyselects the low-voltage cell d as the charge battery cell group insteadof the series of two low-voltage cells a and b.

Lastly, FIG. 35C shows the case where the relation of the voltages isVa>Vb=Vc=Vd. That is, one high-voltage cell a and a series of threelow-voltage cells b to d are connected in series. In this case, theswitching control circuit BT03 selects the high-voltage cell a as thedischarge battery cell group. In addition, the switching control circuitBT03 selects the series of three low-voltage cells b to d as the chargebattery cell group.

On the basis of the determination result shown in the examples of FIGS.35A to 35C, the switching control circuit BT03 outputs the controlsignal Si and the control signal S2 to the switching circuit BT04 andthe switching circuit BT05, respectively. Information showing thedischarge battery cell group, which is the connection destination of theswitching circuit BT04, is set in the control signal Si. Informationshowing the charge battery cell group, which is the connectiondestination of the switching circuit BT05 is set in the control signalS2.

The above is the detailed description of the operations of the switchingcontrol circuit BT03.

The switching circuit BT04 sets the connection destination of theterminal pair BT01 at the discharge battery cell group selected by theswitching control circuit BT03, in response to the control signal Soutput from the switching control circuit BT03.

The terminal pair BT01 includes a pair of terminals A1 and A2. Theswitching circuit BT04 connects one of the pair of terminals A1 and A2to a positive electrode terminal of the battery cell BT09 positioned onthe most upstream side (on the high potential side) of the dischargebattery cell group, and the other to a negative electrode terminal ofthe battery cell BT09 positioned on the most downstream side (on the lowpotential side) of the discharge battery cell group. Note that theswitching circuit BT04 can recognize the position of the dischargebattery cell group on the basis of the information set in the controlsignal Si.

The switching circuit BT05 sets the connection destination of theterminal pair BT02 at the charge battery cell group selected by theswitching control circuit BT03, in response to the control signal S2output from the switching control circuit BT03.

The terminal pair BT02 includes a pair of terminals B1 and B2. Theswitching circuit BT05 sets the connection destination of the terminalpair BT02 by connecting one of the pair of terminals B1 and B2 to apositive electrode terminal of the battery cell BT09 positioned on themost upstream side (on the high potential side) of the charge batterycell group, and the other to a negative electrode terminal of thebattery cell BT09 positioned on the most downstream side (on the lowpotential side) of the charge battery cell group. Note that theswitching circuit BT05 can recognize the position of the charge batterycell group on the basis of the information set in the control signal S2.

FIG. 36 and FIG. 37 are circuit diagrams showing configuration examplesof the switching circuits BT04 and BT05.

In FIG. 36, the switching circuit BT04 includes a plurality oftransistors BT10, a bus BT11, and a bus BT12. The bus BT11 is connectedto the terminal Al. The bus BT12 is connected to the terminal A2.Sources or drains of the plurality of transistors BT10 are connectedalternately to the bus BT11 and the bus BT12. The sources or drainswhich are not connected to the bus BT11 and the bus BT12 of theplurality of transistors BT10 are each connected between two adjacentbattery cells BT09.

The source or drain of the transistor BT10 on the most upstream side ofthe plurality of transistors BT10 is connected to the positive electrodeterminal of the battery cell BT09 on the most upstream side of thebattery portion BT08. The source or drain of the transistor BT10 on themost downstream side of the plurality of transistors BT10 is connectedto the negative electrode terminal of the battery cell BT09 on the mostdownstream side of the battery portion BT08.

The switching circuit BT04 connects the discharge battery cell group tothe terminal pair BT01 by bringing one of the plurality of transistorsBT10 which are connected to the bus BT11 and one of the plurality oftransistors BT10 which are connected to the bus BT12 into an on state inresponse to the control signal S1 supplied to gates of the plurality oftransistors BT10. Accordingly, the positive electrode terminal of thebattery cell BT09 on the most upstream side of the discharge batterycell group is connected to one of the pair of terminals A1 and A2. Inaddition, the negative electrode terminal of the battery cell BT09 onthe most downstream side of the discharge battery cell group isconnected to the other of the pair of terminals A1 and A2 (i.e., aterminal which is not connected to the positive electrode terminal).

An OS transistor is preferably used as the transistor BT10. Since theoff-state current of the OS transistor is low, the amount of charge thatleaks from the battery cell which does not belong to the dischargebattery cell group can be reduced, and reduction in capacity with thelapse of time can be suppressed. In addition, dielectric breakdown isunlikely to occur in the OS transistor when a high voltage is applied.Therefore, the battery cell BT09 and the terminal pair BT01, which areconnected to the transistor BT10 in an off state, can be insulated fromeach other even when the output voltage of the discharge battery cellgroup is high.

In FIG. 36, the switching circuit BT05 includes a plurality oftransistors BT13, a current control switch BT14, a bus BT15, and a busBT16. The bus BT15 and the bus BT16 are provided between the pluralityof transistors BT13 and the current control switch BT14. Sources ordrains of the plurality of transistors BT13 are connected alternately tothe bus BT15 and the bus BT16. The sources or drains which are notconnected to the bus BT15 and the bus BT16 of the plurality oftransistors BT13 are each connected between two adjacent battery cellsBT09.

The source or drain of the transistor BT13 on the most upstream side ofthe plurality of transistors BT13 is connected to the positive electrodeterminal of the battery cell BT09 on the most upstream side of thebattery portion BT08. The source or a drain of the transistor BT13 onthe most downstream side of the plurality of transistors BT13 isconnected to the negative electrode terminal of the battery cell BT09 onthe most downstream side of the battery portion BT08.

An OS transistor is preferably used as the transistors BT13 like thetransistors BT10. Since the off-state current of the OS transistor islow, the amount of charge that leaks from the battery cells which do notbelong to the charge battery cell group can be reduced, and reductionincapacity with the lapse of time can be suppressed. In addition,dielectric breakdown is unlikely to occur in the OS transistor when ahigh voltage is applied. Therefore, the battery cell BT09 and theterminal pair BT02, which are connected to the transistor BT13 in an offstate, can be insulated from each other even when a voltage for chargingthe charge battery cell group is high.

The current control switch BT14 includes a switch pair BT17 and a switchpair BT18. Terminals on one end of the switch pair BT17 are connected tothe terminal B1. Terminals on the other end of the switch pair BT17branch off from two switches. One switch is connected to the bus BT15,and the other switch is connected to the bus BT16. Terminals on one endof the switch pair BT18 are connected to the terminal B2. Terminals onthe other end of the switch pair BT18 extend from two switches. Oneswitch is connected to the bus BT15, and the other switch is connectedto the bus BT16.

OS transistors are preferably used for the switches included in theswitch pair BT17 and the switch pair BT18 like the transistors BT10 andBT13.

The switching circuit BT05 connects the charge battery cell group andthe terminal pair BT02 by controlling the combination of on and offstates of the transistors BT13 and the current control switch BT14 inresponse to the control signal S2.

For example, the switching circuit BT05 connects the charge battery cellgroup and the terminal pair BT02 in the following manner.

The switching circuit BT05 brings a transistor BT13 connected to thepositive electrode terminal of the battery cell BT09 on the mostupstream side of the charge battery cell group into an on state inresponse to the control signal S2 supplied to gates of the plurality oftransistors BT13. In addition, the switching circuit BT05 brings atransistor BT13 connected to the negative electrode terminal of thebattery cell BT09 on the most downstream side of the charge battery cellgroup into an on state in response to the control signal S2 supplied tothe gates of the plurality of transistors BT13.

The polarities of voltages applied to the terminal pair BT02 can vary inaccordance with the configurations of the voltage transformer circuitBT07 and the discharge battery cell group connected to the terminal pairBT01. In order to supply a current in the direction for charging thecharge battery cell group, terminals with the same polarity of theterminal pair BT02 and the charge battery cell group are required to beconnected to each other. In view of this, the current control switchBT14 is controlled by the control signal S2 so that the connectiondestination of the switch pair BT17 and that of the switch pair BT18 arechanged in accordance with the polarities of the voltages applied to theterminal pair BT02.

The state where voltages are applied to the terminal pair BT02 so as tomake the terminal B1 a positive electrode and the terminal B2 a negativeelectrode will be described as an example. Here, in the case where thebattery cell BT09 positioned on the most downstream side of the batteryportion BT08 is in the charge battery cell group, the switch pair BT17is controlled to be connected to the positive electrode terminal of thebattery cell BT09 in response to the control signal S2. That is, theswitch of the switch pair BT17 connected to the bus BT16 is turned on,and the switch of the switch pair BT17 connected to the bus BT15 isturned off. In contrast, the switch pair BT18 is controlled to beconnected to the negative electrode terminal of the battery cell BT09positioned on the most downstream side of the battery portion BT08 inresponse to the control signal S2. That is, the switch of the switchpair BT18 connected to the bus BT15 is turned on, and the switch of theswitch pair BT18 connected to the bus BT16 is turned off. In thismanner, terminals with the same polarity of the terminal pair BT02 andthe charge battery cell group are connected to each other. In addition,the current which flows from the terminal pair BT02 is controlled to besupplied in a direction so as to charge the charge battery cell group.

In addition, instead of the switching circuit BT05, the switchingcircuit BT04 may include the current control switch BT14. In that case,the polarities of the voltages applied to the terminal pair BT02 arecontrolled by controlling the polarities of the voltages applied to theterminal pair BT01 in response to the operation of the current controlswitch BT14 and the control signal Si. Thus, the current control switchBT14 controls the direction of current which flows to the charge batterycell group from the terminal pair BT02.

FIG. 37 is a circuit diagram illustrating configuration examples of theswitching circuit BT04 and the switching circuit BT05 which aredifferent from those of FIG. 36.

In FIG. 37, the switching circuit BT04 includes a plurality oftransistor pairs BT21, a bus BT24, and a bus BT25. The bus BT24 isconnected to the terminal Al. The bus BT25 is connected to the terminalA2. Terminals on one end of each of the plurality of transistor pairsBT21 branch off from a transistor BT22 and a transistor BT23. Sources ordrains of the transistors BT22 are connected to the bus BT24. Sources ordrains of the transistors BT23 are connected to the bus BT25. Inaddition, terminals on the other end of each of the plurality oftransistor pairs are connected between two adjacent battery cells BT09.The terminals on the other end of the transistor pair BT21 on the mostupstream side of the plurality of transistor pairs BT21 are connected tothe positive electrode terminal of the battery cell BT09 on the mostupstream side of the battery portion BT08. The terminals on the otherend of the transistor pair BT21 on the most downstream side of theplurality of transistor pairs BT21 are connected to a negative electrodeterminal of the battery cell BT09 on the most downstream side of thebattery portion BT08.

The switching circuit BT04 switches the connection destination of thetransistor pair BT21 to one of the terminal Al and the terminal A2 byturning on or off the transistors BT22 and BT23 in response to thecontrol signal Si. Specifically, when the transistor BT22 is turned on,the transistor BT23 is turned off, so that the connection destination ofthe transistor pair BT21 is the terminal Al. On the other hand, when thetransistor BT23 is turned on, the transistor BT22 is turned off, so thatthe connection destination of the transistor pair BT21 is the terminalA2. Which of the transistors BT22 and BT23 is turned on is determined bythe control signal Si.

Two transistor pairs BT21 are used to connect the terminal pair BT01 andthe discharge battery cell group. Specifically, the connectiondestinations of the two transistor pairs BT21 are determined on thebasis of the control signal Si, and the discharge battery cell group andthe terminal pair BT01 are connected. The connection destinations of thetwo transistor pairs BT21 are controlled by the control signal Si sothat one of the connection destinations is the terminal Al and the otheris the terminal A2.

The switching circuit BT05 includes a plurality of transistor pairsBT31, a bus BT34, and a bus BT35. The bus BT34 is connected to theterminal B1. The bus BT35 is connected to the terminal B2. Terminals onone end of each of the plurality of transistor pairs BT31 branch offfrom a transistor BT32 and a transistor BT33. One terminal extendingfrom the transistor BT32 is connected to the bus BT34. The otherterminal extending from the transistor BT33 is connected to the busBT35. Terminals on the other end of each of the plurality of transistorpairs BT31 are connected between two adjacent battery cells BT09. Theterminal on the other end of the transistor pair BT31 on the mostupstream side of the plurality of transistor pairs BT31 is connected tothe positive electrode terminal of the battery cell BT09 on the mostupstream side of the battery portion BT08. The terminal on the other endof the transistor pair BT31 on the most downstream side of the pluralityof transistor pairs BT31 is connected to the negative electrode terminalof the battery cell BT09 on the most downstream side of the batteryportion BT08.

The switching circuit BT05 switches the connection destination of thetransistor pair BT31 to one of the terminal B1 and the terminal B2 byturning on or off the transistors BT32 and BT33 in response to thecontrol signal S2. Specifically, when the transistor BT32 is turned on,the transistor BT33 is turned off, so that the connection destination ofthe transistor pair BT31 is the terminal B1. On the other hand, when thetransistor BT33 is turned on, the transistor BT32 is turned off, so thatthe connection destination of the transistor pair BT31 is the terminalB2. Which of the transistors BT32 and BT33 is turned on is determined bythe control signal S2.

Two transistor pairs BT31 are used to connect the terminal pair BT02 andthe charge battery cell group. Specifically, the connection destinationsof the two transistor pairs BT31 are determined on the basis of thecontrol signal S2, and the charge battery cell group and the terminalpair BT02 are connected. The connection destinations of the twotransistor pairs BT31 are controlled by the control signal S2 so thatone of the connection destinations is the terminal B1 and the other isthe terminal B2.

The connection destinations of the two transistor pairs BT31 aredetermined by the polarities of the voltages applied to the terminalpair BT02. Specifically, in the case where voltages which make theterminal B1 a positive electrode and the terminal B2 a negativeelectrode are applied to the terminal pair BT02, the transistor pairBT31 on the upstream side is controlled by the control signal S2 so thatthe transistor BT32 is turned on and the transistor BT33 is turned off.In contrast, the transistor pair BT31 on the downstream side iscontrolled by the control signal S2 so that the transistor BT33 isturned on and the transistor BT32 is turned off. In the case wherevoltages which make the terminal B1 a negative electrode and theterminal B2 a positive electrode are applied to the terminal pair BT02,the transistor pair BT31 on the upstream side is controlled by thecontrol signal S2 so that the transistor BT33 is turned on and thetransistor BT32 is turned off. In contrast, the transistor pair BT31 onthe downstream side is controlled by the control signal S2 so that thetransistor BT32 is turned on and the transistor BT33 is turned off. Inthis manner, terminals with the same polarity of the terminal pair BT02and the charge battery cell group are connected to each other. Inaddition, the current which flows from the terminal pair BT02 iscontrolled to be supplied in the direction for charging the chargebattery cell group.

The voltage transformation control circuit BT06 controls the operationof the voltage transformer circuit BT07. The voltage transformationcontrol circuit BT06 generates a voltage transformation signal S3 forcontrolling the operation of the voltage transformer circuit BT07 on thebasis of the number of the battery cells BT09 included in the dischargebattery cell group and the number of the battery cells BT09 included inthe charge battery cell group and outputs the voltage transformationsignal S3 to the voltage transformer circuit BT07.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, it is necessary to prevent a charging voltage whichis too high from being applied to the charge battery cell group. Thus,the voltage transformation control circuit BT06 outputs the voltagetransformation signal S3 for controlling the voltage transformer circuitBT07 so that a discharging voltage (Vdis) is lowered within a rangewhere the charge battery cell group can be charged.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is less than or equal to that included inthe charge battery cell group, a charging voltage necessary for chargingthe charge battery cell group needs to be ensured. Therefore, thevoltage transformation control circuit BT06 outputs the voltagetransformation signal S3 for controlling the voltage transformer circuitBT07 so that the discharging voltage (Vdis) is raised within a rangewhere a charging voltage which is too high is not applied to the chargebattery cell group.

The voltage value of the charging voltage which is too high isdetermined in the light of product specifications and the like of thebattery cell BT09 used in the battery portion BT08. The voltage which israised or lowered by the voltage transformer circuit BT07 is applied asa charging voltage (Vcha) to the terminal pair BT02.

Here, operation examples of the voltage transformation control circuitBT06 in this embodiment will be described with reference to FIGS. 38A to38C. FIGS. 49A to 49C are conceptual diagrams for explaining theoperation examples of the voltage transformation control circuits BT06for the discharge battery cell groups and the charge battery cell groupsdescribed in FIGS. 35A to 35C. FIGS. 38A to 38C each illustrate abattery control unit BT41. The battery control unit BT41 includes theterminal pair BT01, the terminal pair BT02, the switching controlcircuit BT03, the switching circuit BT04, the switching circuit BT05,the voltage transformation control circuit BT06, and the voltagetransformer circuit BT07.

In an example illustrated in FIG. 38A, the series of three high-voltagecells a to c and one low-voltage cell d are connected in series asdescribed in FIG. 35A. In this case, as described using FIG. 35A, theswitching control circuit BT03 determines the high-voltage cells a to cas the discharge battery cell group, and determines the low-voltage celld as the charge battery cell group. The voltage transformation controlcircuit BT06 calculates a conversion ratio N for converting thedischarging voltage (Vdis) into the charging voltage (Vcha) based on theratio of the number of the battery cells BT09 included in the chargebattery cell group to the number of the battery cells BT09 included inthe discharge battery cell group.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, when a discharging voltage is applied to theterminal pair BT02 without transforming the voltage, an overvoltage maybe applied to the battery cells BT09 included in the charge battery cellgroup through the terminal pair BT02. Thus, in the case of FIG. 38A, itis necessary that a charging voltage (Vcha) applied to the terminal pairBT02 be lower than the discharging voltage. In addition, in order tocharge the charge battery cell group, it is necessary that the chargingvoltage be higher than the total voltage of the battery cells BT09included in the charge battery cell group. Thus, the voltagetransformation control circuit BT06 sets the conversion ratio N forraising or lowering voltage larger than the ratio of the number of thebattery cells BT09 included in the charge battery cell group to thenumber of the battery cells BT09 included in the discharge battery cellgroup.

Thus, the voltage transformation control circuit BT06 preferably setsthe conversion ratio N for raising or lowering voltage larger than theratio of the number of the battery cells BT09 included in the chargebattery cell group to the number of the battery cells BT09 included inthe discharge battery cell group by about 1% to 10%. Here, the chargingvoltage is made higher than the voltage of the charge battery cellgroup, but the charging voltage is equal to the voltage of the chargebattery cell group in reality. Note that the voltage transformationcontrol circuit BT06 feeds a current for charging the charge batterycell group in accordance with the conversion ratio N for raising orlowering voltage in order to make the voltage of the charge battery cellgroup equal to the charging voltage. The value of the current is set bythe voltage transformation control circuit BT06.

In the example illustrated in FIG. 38A, since the number of the batterycells BT09 included in the discharge battery cell group is three and thenumber of the battery cells BT09 included in the charge battery cellgroup is one, the voltage transformation control circuit BT06 calculatesa value which is slightly larger than ⅓ as the conversion ratio N forraising or lowering voltage. Then, the voltage transformation controlcircuit BT06 outputs the voltage transformation signal S3, which lowersthe discharging voltage in accordance with the conversion ratio N forraising or lowering voltage and converts the voltage into a chargingvoltage, to the voltage transformer circuit BT07. The voltagetransformer circuit BT07 applies the charging voltage which is obtainedby transformation in response to the voltage transformation signal S3 tothe terminal pair BT02. Then, the battery cells BT09 included in thecharge battery cell group are charged with the charging voltage appliedto the terminal pair BT02.

In each of examples illustrated in FIGS. 38B and 38C, the conversionratio N for raising or lowering voltage is calculated in a mannersimilar to that of FIG. 38A. In each of the examples illustrated inFIGS. 38B and 38C, since the number of the battery cells BT09 includedin the discharge battery cell group is less than or equal to the numberof the battery cells BT09 included in the charge battery cell group, theconversion ratio N for raising or lowering voltage is 1 or more.Therefore, in this case, the voltage transformation control circuit BT06outputs the voltage transformation signal S3 for raising the dischargingvoltage and converting the voltage into the charging voltage.

The voltage transformer circuit BT07 converts the discharging voltageapplied to the terminal pair BT01 into a charging voltage in response tothe voltage transformation signal S3. The voltage transformer circuitBT07 applies the charging voltage to the terminal pair BT02. Here, thevoltage transformer circuit BT07 electrically insulates the terminalpair BT01 from the terminal pair BT02. Accordingly, the voltagetransformer circuit BT07 prevents a short circuit due to a differencebetween the absolute voltage of the negative electrode terminal of thebattery cell BT09 on the most downstream side of the discharge batterycell group and the absolute voltage of the negative electrode terminalof the battery cell BT09 on the most downstream side of the chargebattery cell group. Furthermore, the voltage transformer circuit BT07converts the discharging voltage, which is the total voltage of thedischarge battery cell group, into the charging voltage in response tothe voltage transformation signal S3 as described above.

An insulated direct current (DC)-DC converter or the like can be used inthe voltage transformer circuit BT07. In that case, the voltagetransformation control circuit BT06 controls the charging voltageconverted by the voltage transformer circuit BT07 by outputting a signalfor controlling the on/off ratio (the duty ratio) of the insulated DC-DCconverter as the voltage transformation signal S3.

Examples of the insulated DC-DC converter include a flyback converter, aforward converter, a ringing choke converter (RCC), a push-pullconverter, a half-bridge converter, and a full-bridge converter, and asuitable converter is selected in accordance with the value of theintended output voltage.

The configuration of the voltage transformer circuit BT07 including theinsulated DC-DC converter is illustrated in FIG. 39. An insulated DC-DCconverter BT51 includes a switch portion BT52 and a transformer BT53.The switch portion BT52 is a switch for switching on/off of theinsulated DC-DC converter, and a metal oxide semiconductor field-effecttransistor (MOSFET), a bipolar transistor, or the like is used as theswitch portion BT52. The switch portion BT52 periodically turns on andoff the insulated DC-DC converter BT51 in response to the voltagetransformation signal S3 for controlling the on/off ratio which isoutput from the voltage transformation control circuit BT06. The switchportion BT52 can have any of various structures in accordance with thetype of the insulated DC-DC converter which is used. The transformerBT53 converts the discharging voltage applied from the terminal pairBT01 into the charging voltage. In detail, the transformer BT53 operatesin conjunction with the on/off state of the switch portion BT52 andconverts the discharging voltage into the charging voltage in accordancewith the on/off ratio. As the time during which the switch portion BT52is on becomes longer in its switching period, the charging voltage isincreased. On the other hand, as the time during which the switchportion BT52 is on becomes shorter in its switching period, the chargingvoltage is decreased. In the case where the insulated DC-DC converter isused, the terminal pair BT01 and the terminal pair BT02 can be insulatedfrom each other inside the transformer BT53.

A flow of operations of the power storage device BT00 in this embodimentwill be described with reference to FIG. 40. FIG. 40 is a flow chartshowing the flow of the operations of the power storage device BT00.

First, the power storage device BT00 obtains a voltage measured for eachof the plurality of battery cells BT09 (step S101). Then, the powerstorage device BT00 determines whether or not the condition for startingthe operation of reducing variations in voltage of the plurality ofbattery cells BT09 is satisfied (step S102). For example, the conditionthat the difference between the maximum value and the minimum value ofthe voltage measured for each of the plurality of battery cells BT09 ishigher than or equal to the predetermined threshold value can be used.In the case where the condition is not satisfied (step S102: NO), thepower storage device BT00 does not perform the following operationbecause voltages of the battery cells BT09 are well balanced. Incontrast, in the case where the condition is satisfied (step S102: YES),the power storage device BT00 performs the operation of reducingvariations in the voltage of the battery cells BT09. In this operation,the power storage device BT00 determines whether each battery cell BT09is a high voltage cell or a low voltage cell on the basis of themeasured voltage of each cell (step S103). Then, the power storagedevice BT00 determines a discharge battery cell group and a chargebattery cell group on the basis of the determination result (step S104).In addition, the power storage device BT00 generates the control signalS1 for setting the connection destination of the terminal pair BT01 tothe determined discharge battery cell group, and the control signal S2for setting the connection destination of the terminal pair BT02 to thedetermined charge battery cell group (step S105). The power storagedevice BT00 outputs the generated control signals S and S2 to theswitching circuit BT04 and the switching circuit BT05, respectively.Then, the switching circuit BT04 connects the terminal pair BT01 and thedischarge battery cell group, and the switching circuit BT05 connectsthe terminal pair BT02 and the discharge battery cell group (step S106).The power storage device BT00 generates the voltage transformationsignal S3 based on the number of the battery cells BT09 included in thedischarge battery cell group and the number of the battery cells BT09included in the charge battery cell group (step S107). Then, the powerstorage device BT00 converts, in response to the voltage transformationsignal S3, the discharging voltage applied to the terminal pair BT01into a charging voltage and applies the charging voltage to the terminalpair BT02 (step S108). In this way, charge of the discharge battery cellgroup is transferred to the charge battery cell group.

Although the plurality of steps are shown in order in the flow chart ofFIG. 40, the order of performing the steps is not limited to the order.

According to the above embodiment, when charge is transferred from thedischarge battery cell group to the charge battery cell group, astructure where charge from the discharge battery cell group istemporarily stored, and the stored charge is sent to the charge batterycell group is unnecessary, unlike in the a capacitive type circuit.Accordingly, the charge transfer efficiency per unit time can beincreased. In addition, the switching circuit BT04 and the switchingcircuit BT05 determine which battery cell in the discharge battery cellgroup and the charge battery cell group to be connected to the voltagetransformer circuit.

Furthermore, the voltage transformer circuit BT07 converts thedischarging voltage applied to the terminal pair BT01 into the chargingvoltage based on the number of the battery cells BT09 included in thedischarge battery cell group and the number of the battery cells BT09included in the charge battery cell group, and applies the chargingvoltage to the terminal pair BT02. Thus, charge can be transferredwithout any problems regardless of how the battery cells BT09 areselected as the discharge battery cell group and the charge battery cellgroup.

Furthermore, the use of OS transistors as the transistor BT10 and thetransistor BT13 can reduce the amount of charge that leaks from thebattery cells BT09 not belonging to the charge battery cell group or thedischarge battery cell group. Accordingly, a decrease in capacity of thebattery cells BT09 which do not contribute to charging or dischargingcan be suppressed. In addition, the variations in characteristics of theOS transistor due to heat are smaller than those of an Si transistor.Accordingly, even when the temperature of the battery cells BT09 isincreased, an operation such as turning on or off the transistors inresponse to the control signals Si and S2 can be performed normally.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Example 1

In this example, an example of the method for forming the conductordescribed in Embodiment 1, and the physical property and characteristicsof the obtained conductor are described.

<Formation of Conductor>

A method for forming the conductor 201 of one embodiment of the presentinvention is described.

First, graphene oxide was prepared. As the graphene oxide, grapheneoxide formed by using flake graphite as a raw material and oxidizing theflake graphite with potassium permanganate and sulfuric acid by themodified Hummers method was used. After a silicon nitride film wasformed over a silicon wafer, a solution in which the graphene oxide wasdispersed into water was applied, and the graphene oxide was observedwith an optical microscope. Many flakes of the graphene oxide had a sizeof approximately 15 m to 50 m, for example. FIGS. 53A to 53C showsexamples of optical micrographs of the graphene oxide.

Next, the graphene oxide was dispersed in a solvent to form grapheneoxide dispersion liquid. 600 ml of water was added to 200 ml ofdispersion liquid in which 3 weight % of graphene oxide was dispersed inwater, and stirring was performed for 12 hours with a stirrer at 600rpm, so that dispersion liquid A was formed.

Next, a graphene compound sheet (represented by GO-1) was formed usinggraphene oxide dispersion liquid as a raw material by a spray drymethod. Here, GO-1 was formed on a wall surface of a chamber of a spraydry apparatus. The following shows details.

As the spray dry apparatus, a mini spray dryer B-290 manufactured byNihon BUCHI K.K. was used. An inlet was set to 160° C. It is consideredthat a nozzle and the vicinity thereof were heated to a temperaturehigher than or equal to 100° C. The dispersion liquid A was supplied tothe nozzle of the spray dry apparatus at a rate of approximately 65ml/minute. The dispersion liquid A was supplied from the nozzle to thechamber in the form of mist together with a nitrogen gas at a flow rateof 60 L/min.

Part of the dispersion liquid A supplied to the chamber in the form ofmist was collected to a collection container as powder of the grapheneoxide, and the other part was formed as GO-1 on an inner wall of a wall281 a of a cylindrical chamber. FIG. 41 is a photograph of GO-1 formedon the inner wall.

Next, GO-1 was peeled from the inner wall of the chamber. GO-1 includesa plurality of sheets of graphene oxide overlapping with each other. Theaverage of the thicknesses of GO-1 was 8.6 m. Ten points in a region ofapproximately 10 cm square in GO-1 were measured, and the average wascalculated. GO-1 which was obtained was subjected to XRD evaluation andelectrical conductivity measurement. The results are shown later. FIG.42 is an example of a photograph of GO-1. FIG. 54A is an opticalmicrograph of GO-1, and FIG. 54B is an SEM image thereof.

Next, GO-1 was subjected to reduction treatment to obtain conductors.Here, as conditions for the reduction treatment, three conditions wereemployed. In the obtained conductors, a conductor which was subjected toonly thermal reduction at 250° C. is referred to as RGO-1A, a conductorwhich was subjected to only thermal reduction at 300° C. is referred toas RGO-1B, and a conductor which was subjected to chemical reduction andthen subjected to thermal reduction at 250° C. is referred to as RGO-1C.FIG. 43 is an example of a photograph of RGO-1C. FIGS. 55A and 55B areoptical micrographs of RGO-1A and the RGO-1C. FIGS. 56A and 56B are SEMimages of RGO-1A and RGO-1C.

Graphene oxide included in GO-1 was reduced by the reduction treatment,and thus the oxygen content was reduced. RGO-1A to RGO-1C each include agraphene compound. RGO-1A to RGO-1C are each a graphene compound sheet.

Conditions for chemical reduction are described. Ascorbic acid was usedas a reducing agent, and an ethanol water solution at a concentration of80% was used as a solvent. 0.3375 g of ascorbic acid and 0.078 g oflithium hydroxide were added to 100 ml of an ethanol water solution toform a reducing solution.

GO-1 which was obtained was put in the reducing solution, and reductionwas performed at 60° C. for 3 hours. After that, washing was performedwith ethanol.

Next, conditions of the thermal reduction are described. Heat treatmentwas performed at 250° C. in a reduced-pressure atmosphere (1 kPa) for 10hours. Through the above process, RGO-1A to RGO-1C of sheet-likeconductors were obtained.

<Evaluation of Conductor or the Like>

Next, evaluation results of the physical properties and characteristicsof GO-1 and RGO-1A to RGO-1C are described.

The electrical conductivities of the obtained samples were measured. Forthe measurement of the electrical conductivities, Loresta GP (MCP-T610)manufactured by Mitsubishi Chemical Analytech, Co., Ltd. was used.MCP-TP06P was used for a probe, and the measurement was performed atroom temperature.

The electrical conductivities of GO-1, RGO-1A, RGO-1B, and RGO-1C were0.0021 S/cm, 4.2 S/cm, 6.3 S/cm, and 25 S/cm, respectively.

Next, XRD evaluation was performed. The XRD evaluation results of GO-1,RGO-1A, and RGO-1B are shown in FIG. 44A, FIG. 44B, and FIG. 44C,respectively. As each of RGO-1A and RGO-1B, two samples were fabricatedunder the same conditions. The samples of RGO-1A and the samples ofRGO-1B were fabricated in different lots. The evaluation results areshown by a solid line and a dashed line in FIGS. 44B and 44C.

Peaks were observed at around 9°, around 22°, and around 23° in in FIG.44A, FIG. 44B, and FIG. 44C, respectively, and thus it is suggested thatthe average values of the interlayer distances in GO-1, RGO-1A, andRGO-1B are approximately 0.98 nm, approximately 0.40 nm, approximately0.39 nm, respectively. Here, the term “around” means±0.7° or ±0.5° of anangle “2θ” measured by XRD evaluation, for example. That is, “around 9°”is, for example, greater than or equal to 8.3° and less than or equal to9.7°, or greater than or equal to 8.5° and less than or equal to 9.5°.

Next, XPS analysis was performed. First, the result of GO-1 is shown.

The quantification values (unit: atomic %) of elements of carbon,oxygen, sulfur, and nitrogen in GO-1 were 63.7, 33.5, 2.3, and 0.6,respectively.

FIG. 45A, FIG. 45B, FIG. 46A, and FIG. 46B show a C1s spectrum ofcarbon, an O1s spectrum of oxygen, a S2p spectrum of sulfur, and a N1sspectrum of nitrogen, respectively.

Next, the XPS analysis results of RGO-1A and RGO-1C are shown.

The quantification values (unit: atomic %) of elements of carbon,oxygen, sulfur, and nitrogen in RGO-1A were 80.7, 15.8, 1.4, and 2.1,respectively.

The quantification values (unit: atomic %) of elements of carbon,oxygen, sulfur, and nitrogen in RGO-1C were 89.5, 9.2, 0.2, and 1.2,respectively.

FIG. 47A, FIG. 47B, FIG. 48A, and FIG. 48B show a c1s spectrum ofcarbon, an O1s spectrum of oxygen, a S2p spectrum of sulfur, and a Nsspectrum of nitrogen, respectively, of RGO-1A and RGO-1C.

Example 2

In this example, a method for forming the conductor of one embodiment ofthe present invention and the physical property and characteristicsthereof are described.

<Treatment with Solvent>

A solvent was applied to GO-1 of the graphene compound sheet obtained inExample 1. Specifically, NMP was dropped over GO-1 and then appliedusing a blade with a gap of 100 m. After that, the solvent wasvolatilized (a graphene compound sheet obtained by the treatment withthe solvent is referred to as GO-2). As the solvent, NMP was used.

<Reduction>

Next, GO-2 was subjected to reduction treatment. As conditions for thereduction treatment, two conditions were employed. A conductor which wassubjected to heat treatment at 250° C. for 10 hours is referred to asRGO-2A, a conductor which was subjected to heat treatment at 300° C. for10 hours is referred to as RGO-2B.

<Evaluation of Conductor or the Like>

Next, evaluation results of the physical properties and characteristicsof GO-2, RGO-2A, and RGO-2B are described.

The electrical conductivities of RGO-2A and RGO-2B were 86 S/cm and 101S/cm, respectively. The electrical conductivity of GO-2 was lower thanor equal to the lower measurement limit.

The XRD evaluation results of GO-2 and RGO-2A are shown in FIGS. 49A and49B.

A peak at around 8° and a broad peak at around 16° are observed in FIG.49A. It is suggested that the interlayer distances at the peaks in GO-2are approximately 1.1 nm and approximately 0.50 nm.

A peak was observed at around 25° in FIG. 49B, and thus it is suggestedthat the average of the interlayer distances in RGO-2A is approximately0.36 nm.

FIG. 50A shows FT-IR evaluation results of GO-1 formed in Example 1 andGO-2 formed in this example. The evaluation result of GO-1 is shown by adashed line, and the evaluation result of GO-2 is shown by a solid line.In the FT-IR, the measurement was performed by attenuated TotalReflection, and cadmium telluride mercury detector was used as adetector.

As shown in FIG. 50A, in GO-1, peaks were observed in ranges ofwavenumbers of 1250 cm⁻¹ to 1000 cm⁻¹, 1620 cm⁻¹ to 1680 cm⁻¹, and 1750cm⁻¹ to 1650 cm⁻¹, which are considered to show a C—O bond, a C═C bond,and a C═O bond, respectively. In GO-2, the ratio of the peak intensityand the peak position were different from those in GO-1, and a slightlystrong peak was observed at around 1640 cm⁻¹. Furthermore, a peak wasobserved at around 1045 cm⁻¹ in GO-1. This peak may be derived from asulfur compound, for example.

The FT-IR evaluation results suggest that in GO-2 which was subjected todrying after application of NMP, changes in the kind of a functionalgroup and the concentration, such as a reduction in a C—O bond, wereproduced. Accordingly, there is a possibility that the interlayerdistance was increased. Furthermore, there is a possibility that anintercalation compound was formed by existence of an element such asnitrogen between layers. FIG. 50B is an enlarged view of FIG. 50A ataround 3000 cm⁻¹ in GO-2. Peaks were observed at around 2880 cm⁻¹ andaround 2950 cm⁻¹, which are considered to show a C—H bond. It isconsidered that there is a possibility that H in NMP is related, forexample.

Example 3

In this example, a storage battery including RGO-1A and RGO-1B formed inExample 1 in an electrode was manufactured.

<Manufacture and Evaluation of Storage Battery>

A storage battery was manufactured using RGO-1A or RGO-1B in anelectrode and using a lithium metal in a counter electrode. Thecharacteristics were measured with the use of a CR2032 coin-type storagebattery (with a diameter of 20 mm and a height of 3.2 mm). As aseparator, polypropylene was used. An electrolyte solution was formed insuch a manner that lithium hexafluorophosphate (LiPF₆) was dissolved ata concentration of 1 mol/L in a solution in which ethylene carbonate(EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 1:1. Apositive electrode can and a negative electrode can were formed ofstainless steel (SUS).

Next, charging and discharging of the manufactured storage battery wereperformed. The measurement temperature was 25° C. Constant currentcharging was performed at a current density per weight of 30 mA/g withthe upper voltage limit set to 4.8 V and constant current dischargingwas performed at a current density per weight of 30 mA/g with the lowervoltage limit set to 2 V.

The charge and discharge characteristics of RGO-1A and RGO-1B are shownin FIGS. 51A and 51B. Samples represented by solid lines in FIGS. 51Aand 51B correspond to the samples represented by the solid lines inFIGS. 44B and 44C, and samples represented by dashed lines in FIGS. 51Aand 51B correspond to the samples represented by the dashed lines inFIGS. 44B and 44C. The vertical axis represents voltage, and thehorizontal axis represents capacity per weight of RGO-1A or RGO-1B.

As shown in FIGS. 51A and 51B, favorable charge and dischargecharacteristics were obtained.

Here, it is considered that conductors such as RGO-1A and RGO-1Bcontribute to oxidation-reduction reaction in charging and discharging.As a reason of high capacity, there is a possibility that anions andcations were intercalated into and deintercalated from the conductor 201in charging and discharging of a power storage device. There is apossibility that oxidation-reduction reaction occurred by intercalationand deintercalation of anions or cations. As the anion and the cation, alithium ion or an ion other than a lithium ion included in anelectrolyte solution can be given. Alternatively, it is considered thatthere is a possibility that decomposition product of an electrolytesolution, such as Li₂CO₃ or Li₂O, was precipitated on a surface of thesheet-like conductor and an electrochemical reaction by a peroxide ionoccurs.

Example 4

In this example, an electrode which includes GO-1 formed in Example 1 asa current collector and is described in Embodiment 2 as an example wasformed.

[Synthesis of Lithium-Manganese Composite Oxide]

Using any of the active materials described in Embodiment 2 as a rawmaterial, Li₂CO₃, MnCO₃, and NiO were mixed at a ratio ofLi₂CO₃:MnCO₃:NiO=0.84:0.8062:0.318. After that, baking was performed at1000° C., so that Sample A which is a lithium-manganese composite oxideincluding nickel was fabricated.

<Coating>

Next, graphene oxide dispersion liquid was formed. Water was used as asolvent. The concentration of graphene oxide was 2 weight %.

300 g of the obtained Sample A was added to the graphene oxidedispersion liquid in which the content of graphene oxide is 6 g andmixed to obtain Solution A. After that, heat treatment was performed onSolution A at 50° C. under reduced pressure, so that Mixture B wasobtained.

Next, Mixture B which was subjected to the heat treatment was added to areducing solution, and heat treatment was performed at 60° C. for threehours, so that Solution C was obtained. In the reducing solution,ascorbic acid was used as a reducing agent, an ethanol water solution ata concentration of 80 vol % was used as a solvent, lithium hydroxide ata concentration of 3.90 weight % was added to Sample A, and theconcentration of the ascorbic acid was 16.87 weight % with respect toSample A.

Next, Solution C was collected with a centrifuge to obtain Mixture D.After that, Mixture D was ground in an alumina mortar, and Mixture Dwhich was ground was subjected to heat treatment at 170° C. under areduced pressure for 10 hours to obtain Sample E. Sample E includes alithium-manganese composite oxide. The lithium-manganese composite oxideincludes nickel. A surface of the lithium-manganese composite oxide iscoated with a graphene compound.

<Formation of Electrode>

Next, Sample E, AB, PVDF, and NMP were mixed to form Mixture F. InMixture F, the compounding ratio of Sample E:AB:PVDF was set to 90:5:5(weight %). Next, Mixture F to be the layer 102 was applied to GO-1.After that, heat treatment was performed at 80° C. for 30 minutes, sothat the NMP was volatilized. After that, an electrode was pressed witha roller press machine. After that, heat treatment was further performedat 250° C. for 10 hours, so that Electrode G including the layer 102 wasformed over GO-1 which was subjected to the heat treatment. Thedescription of the conductor 201 described in Embodiment 1 as an examplecan be referred to for GO-1 which was subjected to the heat treatment.GO-1 which was subjected to heat treatment is referred to as theconductor 201.

Furthermore, Electrode H was formed in such a manner that Mixture F wasapplied to aluminum foil, and then the NMP was volatilized, and pressingand the subsequent heat treatment were performed under conditionssimilar to those of Electrode G.

<Tem Observation>

FIG. 57 is a TEM photograph of a cross section of Electrode G. FIG. 58is a photograph of Electrode G observed at higher magnification.

(Evaluation of Electrode)

A storage battery was manufactured using obtained Electrodes G and H. Alithium metal was used for a counter electrode. The characteristics weremeasured with the use of a CR2032 coin-type storage battery (with adiameter of 20 mm and a height of 3.2 mm). As a separator, polypropylenewas used. An electrolyte solution was formed in such a manner thatlithium hexafluorophosphate (LiPF₆) was dissolved at a concentration of1 mol/L in a solution in which ethylene carbonate (EC) and diethylcarbonate (DEC) were mixed at a volume ratio of 1:1. A positiveelectrode can and a negative electrode can were formed of stainlesssteel (SUS). Here, the weight ratio of Sample E and the conductor 201 inElectrode G per unit area was 1:0.54.

Next, charging and discharging of the manufactured storage battery wereperformed. The measurement temperature was 25° C. Constant currentcharging was performed at a current density per Sample E weight of 30mA/g with the upper voltage limit set to 4.8 V and constant currentdischarging was performed at a current density per weight of 30 mA/gwith the lower voltage limit set to 2 V.

The charge and discharge characteristics of the storage batteryincluding Electrode G and Electrode H are shown in FIGS. 52A and 52B.Electrode G is shown by a solid line, and Electrode H is shown by adotted line. FIG. 52A shows charge and discharge curves in a firstcycle, and FIG. 52B shows charge and discharge curves in a tenth cycle.The vertical axis represents voltage, and the horizontal axis representscapacity normalized by weight of Sample E.

In FIG. 52B, discharge capacities of Electrodes G and H were 322 mAh/gand 259 mAh/g, respectively. The maximum discharge capacity of theconductor 201 (which corresponds to RGO-1A) was 48 mAh/g. Accordingly,the maximum contribution obtained by the reaction of the conductor 201with respect to the whole discharge capacity can be estimated to be48×0.54=30 [mAh/g]. Accordingly, the capacity of Electrode G isestimated to be higher than that of Electrode H by 30 mAh/g. However,the result of FIG. 52B shows that the capacity of Electrode G is higherthan that of Electrode H by 63 mAh/g. Accordingly, the capacity of theconductor 201 might be increased with the increasing number of chargeand discharge cycles. Alternatively, the conductor 201 and Sample E, AB,or PVDF might be reacted by charge and discharge, or the reaction withNMP used in the formation of Electrode G might make a contribution.

Next, Mixture F was applied to a sheet 1103 which was covered with anundercoat over an aluminum current collector to form a layer 1102 inorder to investigate reaction between the layer 102 and the conductor201. After that, GO−1 was placed over the layer 1102 and subjected toheat treatment at 250° C. GO-1 which was subjected to the heat treatmentis referred to as a conductor 1201.

After that, the conductor 1201 was peeled off. FIG. 59 is a photographof the layer 1102 and the conductor 1201 after the peeling. A regionwhere the conductor 1201 overlapped with the layer 1102 is referred toas area 1, and a region where the conductor 1201 did not overlap withthe layer 1102 is referred to as area 2.

FIGS. 60A and 60B show XPS measurement results of the area 1 and thearea 2. FIG. 60A shows S2p spectra of sulfur, and FIG. 60B shows Mn2pspectra of manganese.

In the region which overlapped with the conductor 1201, a peakindicating existence of a sulfur compound was detected. Theconcentration of sulfur in the area 1 which was detected by XPS was 5.7atomic %, and the concentration of sulfur in the area 2 which wasdetected by XPS was 0 atomic %.

Furthermore, in the area which overlapped with the conductor 1201,almost no peak indicating existence of manganese was detected.Accordingly, there is a possibility that a layer including sulfur wasformed over the layer 1102 in the area 1. The concentration of manganesein the area 1 which was detected by XPS was 0 atomic %, and theconcentration of manganese in the area 2 which was detected by XPS was6.0 atomic %. Sulfur included in GO-1 might be precipitated at aninterface between GO-1 and the layer 1102, which suggests a possibilitythat a reaction occurred between the conductor 1201 and the layer 1102.

This application is based on Japanese Patent Application serial no.2015-128002 filed with Japan Patent Office on Jun. 25, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for forming a conductor, wherein a sheethaving a thickness of greater than or equal to 50 nm and an area ofgreater than or equal to 1 mm² is formed by stacking a plurality ofsheets of graphene oxide so as to partly overlap with each other and,wherein the sheet is subjected to reduction treatment.
 2. The method forforming a conductor according to claim 1, wherein a concentration ofcarbon is higher than 80 atomic %, a concentration of oxygen is higherthan or equal to 2 atomic % and lower than or equal to 20 atomic %, andan interlayer distance is greater than or equal to 0.335 nm and lessthan or equal to 0.7 nm.
 3. A method for forming a conductor, comprisingsteps of: supplying a dispersion liquid including a graphene oxide to achamber from a nozzle; forming a mist of the dispersion liquid in thechamber; heating the mist; and depositing a graphene compound sheet overa substrate on a wall surface of the chamber.
 4. The method for forminga conductor according to claim 3, wherein air in the chamber issuctioned.
 5. The method for forming a conductor according to claim 3,wherein a part of the graphene oxide of the dispersion liquid iscollected to a collection container through the chamber.
 6. The methodfor forming a conductor according to claim 3, wherein the substrate ismoved while depositing the graphene compound sheet.
 7. The method forforming a conductor according to claim 3, wherein the nozzle is movedwhile depositing the graphene compound sheet.
 8. The method for forminga conductor according to claim 3, wherein an interlayer distance in thegraphene compound sheet is greater than 0.8 nm and less than or equal to2 nm.
 9. The method for forming a conductor according to claim 3,wherein the graphene compound sheet has a peak in a range of greaterthan or equal to 7° and less than or equal to 10 in XRD evaluation. 10.The method for forming a conductor according to claim 3, wherein atemperature of the heating is higher than or equal to 40° C. and lowerthan or equal to 70° C.
 11. The method for forming a conductor accordingto claim 3, further comprising reducing the graphene compound sheet. 12.The method for forming a conductor according to claim 11, wherein thereduced graphene compound sheet has a peak in a range of greater than orequal to 210 and less than or equal to 27 in XRD evaluation.
 13. Themethod for forming a conductor according to claim 11, wherein aconcentration of carbon of the reduced graphene compound sheet is higherthan 80 atomic %, a concentration of oxygen of the reduced graphenecompound sheet is higher than or equal to 2 atomic % and lower than orequal to 20 atomic %.